Thats what the error message says when I try leaving Kuldahar through the blue portal in the roots of the big tree. I got the patch and tried again, but no difference... what do I do?

Assertion failed messages generally occur when some game charecter or PC is in a position where it should not be (eg: inside a tree ;) ).This will happen whenever the game trys to save your game in such a case.

You might try killing those buggers south of the portal before you exit.
Else approach the portal from a different direction.
If all else fails, go to previous save n reload.

Tried it all...still not working :(

I tried all three saves I had, i.e, Quick Save Backup, Quick Save and AutoSave...neither works, and I don't have an older save...

I uninstalled the game, deleted the folder manually and reinstalled the game. Substituting my save then and trying to load the next area again brought up the same error message...

Any idea what is going on?