well this if something is weird.
My sir anomen is level 40.
I got TDD and TOB
but now he gets a level up...
Anomen is level 41 and WHAT!!
He can't cast any priest spells...
I mean that when he gets that level up all his spells disappear!
BUG!?!?!

That is usually the result of an unsafe xp cap remover.
I think there is a safe cap remover at baldurdash. Hope you have an "safe" save game to restore from, because I am pretty sure the level up effects are irrevocable.

Yes i got a safe save game [img]smile.gif[/img]
HUH

But my thief is level 43 and nothing weird has happened...

I do not tamper with the xp/level cap myself as I consider it cheating of highest degree, so I can only relate what I have heard from other sources. This particular bug is usually seen because some of the original .2da files in the game contain information on level 41 and others (such as spell advancement) does not. I cannot tell if this is your problem with certainty.