It's been years since I last played BG2 and I wanted to play it again today. However, after a reinstall of the game (due to "CD not found-errors" and the widescreen-mod (and the bonus merchant-files although I'm not sure that was really needed), not only do I have black bars replacing the bottom and right UI-bars but when I hover over those parts, the cursor starts dragging all over it, see attachments.

I didn't have this issue when I played the game (briefly) last time and I've tried some suggested solutions even though they might not have been completely related but it didn't solve the problem for me unfortunately. My chosen resolution was my native one of 1920x1080. Any idea how I can solve this?

I tried to Google the issue but have found no luck.

Hi. Sorry for the delay of my answer. Assuming that you've already setup the game in compatibility mode and run it as administrator, here's what i would do at your place:
I would remove completely the game, even the folder manually. Then I would provide a clear and new installation: before start the game, I would check every option double clicking bgconfig.exe, clicking on "apply" at the end and playing shortly the game without any mod to check if everything is alright, quicksaving as soon as possible.
Then, and only then, if everything works fine, I would install some mod.
If the graphical glitch of the cursor is still there, you should try to disable acceleration 3d in bgconfig.exe.