Had something similar happen to me some time ago, it seems some sort of user permission error and despite my effort to correct it, I just gave up and ended reinstalling Windows.

Same as you, I could access pretty much all my files from Linux just fine but running a new Windows user account just gave me all kinds of weird simptoms.
Note that you can't access "my documents" folder from another Windows account without change file permissions (as far as I know), once you do it the old account is as good as extint.

I can't recall what triggered the errors but I like to atribute the fault to some corrupt file. Was very tedious trying to solve the problem, including virus scan wich was improbable but did it anyway, and some Registry entries were un-modifiable without reason.

Note that you don't need to manually reactivate Windows 10 in most cases, it kinda reactivates itself once you connect to internet. From what I know about the procees, M$ creates a unique key of your hardware (motherboard?) and as soon as you connect to the internet on your freshly installed Windows, it will look at M$ database for that key and if it's valid, it will just reactivate itself.

Grub is Scary because it's doing a lot of things while balanced on a 20 year old codebase. It's very opaque not desirable in this modern age of EFI/MBRless booting. You don't even have to change it, as ReFind is installed a level higher. And if you prefer to try other things, there's Systemd-boot, and Limine; all of which aim to provide a far better booting experience than Grub.

I'm not advocating one over another but come on. Basically every distro uses GRUB. And code doesn't get worse with age. It's correct or it's not.

I guess SystemD makes sense if you have EFI since it's going to be on your system already. No need to install something extra that does the same thing really.

Code may not get worse with age in the traditional sense, but the changing standards, formats, and structures of drive handling do change. More to the point, just because something is a standard because of momentum doesn't make it a good thing. Gnome is a screaming example of this.

And put simply, Grub has too many fiddly bits that could go wrong.

Such as? I mean are you just saying it's too complex for you? If so that's fine. But I would not say it's not fiddly. You set it up correct or you don't. But it has a lot of features that can be useful.
In any case I guess it doesn't matter as the issue at hand here is Windows.

Please, look out. I think I must warn you again:

Changing Windows 10 file permissions causes a lot of weird behaviors indeed, as DarkArt pointed out:

This is because many programs and applications check file and user account permissions to function. Therefore, for example, if one program or file is tied to your first user account, your new created account may not be able to open this program and many things might stop working.

Changing file permissions on Windows also takes a terrible amount of time.

Please, be warned of that too.

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On a side-topic, I could give you videos and tutorials on how to create a custom Windows 10 ISO to debloat it of anything that isn't useful, if you end up reinstalling your system.
A program for that that is good and easy to use, that require no CLI knowledge at all is called WinToolKit.
You use official Microsoft ISOs, downloaded from their website, and literally edit the ISO to remove Microsoft store, Cortana, Windows Antivirus, useless programs, or whatever package that comes installed you want really.
You can even deactivate automatic updates (Which, allegedly hurt your system and removes user freedom through time, even if they release security updates now and then...).

No, I mean it does too many things, has too many moving parts, and has no need to have the codebase complexity it does. Most of it is legacy support for formats that are long since dead.

It is very Un-Unix.

If you are still able to boot into Linux and the issue began with an Linux update I would suggest you go to the Software Manager (left sidebar) and input "Boot Repair" without quotes. Install and run Boot Repair. Do not do anything once you start the repair process unless prompted to do so by the repair program. LET IT FINISH before doing anything to interrupt the process that can take some time. After the repair is finished, give windows a try.

I've used this program on several Linux Mint dual boot systems including both of my own over the past few years. It worked perfectly on LM20 for me. I've not used it on my current install of LM21.1 but I would if I had the problems you describe.

Even so, use the program at your own risk as not actually seeing your system makes my solution a possible fail. I have used the program to fix similar problems but offer no guarantees. Good luck. =)

Hope you get it fixed.

Well, I've gotten a lot of responses since I checked earlier today. Thanks a lot, and much of it is interesting, but to address the most relevant issues:

EverNightX, thanks for the administrative setting fix. I'll look into that.

Dark_art_, I'll avoid the my documents folder, though I really never use it, anyway. The thing is, I never connect Windows 10 to the Internet, because I despise its telemetry, so I'm doing this without using an Internet connection.

Darvond and EverNightX, I think you both make good points, but I'm not sure GRUB is actually the issue, since both Windows and Linux are booting (Windows at least to the point that it's being chosen by GRUB, so it's my understanding that whatever happens after that has nothing to do with GRUB).

Keys, I really have no intention of going into all of my files and changing their permissions, but rather just changing settings for the user profile itself. As for all that other stuff about cleaning up everything, I could look into that, but note that I don't get any Windows updates anyway, since it's offline.

Stuff, so you're saying that a program running in Linux can fix the problem in Windows? Interesting, and I'll look into that too, because it may well be the best bet, but I'll see. Thanks. And I'll be careful and read about it and what it actually does, so that hopefully I'll understand what I'm doing. In any case, I'll back up everything first, but I really want to just handle this in the easiest way possible. I mean, obviously something changed, which can somehow just be chanced back to whatever it was before, so that's what I'd like to do.

Correct, this has nothing to do with GRUB. GRUB only alters the first first boot step(s), and from what I understand you are able to boot far past the initial boot steps and into where Windows is past drivers and into the profile.

If it was GrUB you'd most likely not be able to actually handle the boot process over to Windows in the first place.

I've had something similar just because I accessed the Windows partition (won't matter if it's on a different HDD at all in this case), not entirely sure if Mint actually changes something or it's a bit-flip in the process, but regardless, Windows sometimes get a fit (although rarely, luckily) when I boot up Windows again. (I'm leaning towards something with last accessed time stamps.)

What's weird, I don't recall having that much of problem with XP or on Windows 7, at least not that frequent, relatively.

In any case, I've found out that running SFC /scannow and DISM helps as in some cases. And messing with permissions on Windows is almost a death sentence, but most problems here would be solved by creating a new userprofile and booting into that one. If that works you just delete the old one.

net user test /add
net localgroup administrators test /add

Worst case, if it''s a physical corruption on the HDD you might try, either in Win or on Linux to try repairing it.

If none of the hint in this thread works then yes, you might have to reinstall Windows, and then make GRUB reinit again.

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If gaining access to the advanced recovery gives you a problem, try this;

First on Linux:
rename \windows\system32\utilman.exe to f.ex. utilman.exe.bak
copy \windows\system32\cmd.exe to \windows\system32\utilman.exe

Boot normally into Windows.

At the Login Screen click the EASE OF ACCESS icon this should launch a CMD window.

Type in:
net user test /add
Then:
net localgroup administrators test /add

Close the CMD prompt. Click the Power icon (bottom right corner of the screen) and select RESTART. Lastly, sign in as test without a password.
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Depending of how this works you might have to do some trial and error, same goes with repairing the boot process.

No, I'm saying that if you have a dual boot setup with Linux Mint/Windows where you get a black screen on boot with the options of booting to Linux or Windows and . . . after a LM update . . . you can boot to LM but NOT to Windows after selecting the Windows boot option . . . you may simply have a corrupted GRUB install.

The "Boot Repair" program available on Linux Mint for free will look at the GRUB install, offer to download the latest version or use the existing version to repair anything that has been corrupted in the install. It's been more than a year since I used it but it fixed the near identical problem I was having and later my brothers install . . couldn't boot to Windows after a Linux update.

IF . . your setup does not start with a black screen with options to boot to Linux or windows or repair, etc than your setup is different from mine and you probably have a "Windows" problem. Ignore my post. What I am describing is the GRUB install that usually occurs when dual booting with Linux/XXXX can get corrupted due to an Linux update/upgrade/interference during and update which will cause the code to boot to X or X to fail. The Boot Repair program can only fix THAT problem and not a Windows problem. If you have a corrupted Windows program it will not. So if both were working just fine then . . after a Linux update . . . you suddenly cannot choose to boot to Windows successfully . . then you may have the GRUB corruption which breaks the boot process. If Windows crashed while you were using it then you definitely have a Windows problem.

I would find my Windows product key and reinstall it on a fresh NTFS partition. AFter backing up my important files through Linux.

Fixing Windows installations and troubleshooting is a royal pain, with limited logs, information or tools. I would just give up as the problem seems really extensive on your end.

I would also consider just using separate physical drives for Linux and Windows in the future, SSDs are dirt cheap these days.

A bit unrelated, but GRUB is the devil. See recent events like bricking your Arch installation after an update. Having to rescue your OS by chrooting from a live ISO and manually fixing your GRUB efi setup is a royal pain.

Did I mention royal pain enough?

I think the bigger problem being how little space you have to work with assuming the minimum blocksize to get the system up and running, and Grub unless configured right will just stall and do nothing with no recourse to fix it.

Once set up Grub is fine, but any changes need to be carefully done to boot right, best to have a backup rescue disc, thumbdrive, or whatnot in order to edit settings and/or revert to the previous state in the event your kernel update or whatever doesn't work.

sanscript, thanks for the tips; I'll make a new user with the administrator privileges, and try the later stuff if that doesn't work, but I suspect it might.

Stuff, I don't think it's a GRUB problem at all, but that's all good to know, thanks.

rojimboo, I'm trying to avoid having to reinstall if I don't need to, and since I haven't even really tried changing anything at all yet, I think I'll do that first, because it can't make things any worse, and then if that doesn't work I'll reinstall. I just have to back up some stuff before I do anything. And I don't use SSD because I don't like how it can't be erased without erasing the entire thing (like I couldn't erase a single partition and wipe the data, because of the stupid wear leveling garbage).

rtcvb32, for what it's worth, I've never had a serious problem with GRUB. I had a few difficulties at first relating to the installation and updating of it, to recognize all of the partitions correctly, but that's just because I didn't yet understand how it works exactly.