I finished BG1 part (I think), then talked to Belt in the castle. I assume that it would start BG2, however it just crashes the game. It attempts to load a new map, then only black screen shows up. I cannot even open a menu!

Please help.

When you say "crashes" do you mean you get an error message or that the screen just stays black?
How long did you wait? It can stay black for a really long time and then begin BG2 - there's a lot of info it has to transfer over (importing not only your character but Jaheira/Minsc/Imoen etc)
Give it 5 minutes - go make a cup of tea (though you might then miss the cut scene - so make the cup of tea beforehand and sit there drinking it during the wait)

That works! Thank you.

Glad it's ok :)
(I remember thinking it had stuck too, the first time I played it)

This may just be specific to me and my systems but every time I ever tried using BGT (instead of Tutu) I had all sorts of problems. Even after it was patched up and supposed to be relatively fixed I still ran into game-breaking crashes and such. So I no longer bother with BGT as Tutu accomplishes the same thing with no problems and is more user-friendly in my experience.

@TrollumThinks: There really should be a 4th wall breaking moment where it notifies you to "please take 5, this will take a while" before it starts that processing you described. plus, and this is important, for your BG2 game you can eliminate ALL the mods that specifically address BG1 content.
Fewer mods = fewer problems

Yeah, there should be - I think they mention it in the readme (and we all read the readmes, don't we? LOL )

I'm not sure whether it was possible to add a 'loading' type screen to the process but they could've had some humour with it and had Duke Belt tell you in advance: "You may find yourself suspended in limbo during the unusually long teleport spell for, oh, about 5 minutes. Anybody observing you from the outside would be better off going for a cup of tea." or something.

It's not actually that bad, once you know it's supposed to do that.

As for other bugs/problems - I've never encountered anything serious (even any non-serious bugs don't spring to mind so must be minor if there) and my game is medium-to-heavily modded. Then again, my BG1-side of the mods adds only Unfinished Business, Mini-Quests and Encounters and the BG1 NPC Project.

that would be both hilarious and awesome