@OP, Alt-Tab should work by now, try it.

Regarding Manage Group, this actually looks very odd. Please do the following:
1) Start a new game, enter Party manager
2) Close it again, end the game
3) In Steam, rightclick the game name and choose "Properties -> Local Files -> Browse local files"
4) Move to Folder "schick_data" and find the file "output_log.txt"
5) post that file here, if it is too big, zip it before posting

That would help pinpointing this error. But from the looks of it, there might be corrupted files involved, so maybe you could also solve this by Steam -> Local Files -> Verify local game cache

According to the output_log-File, beorn001.dsa1char causes problems for some reason. If you could zip those chars and send them to support@realmsorarkania.com , we can surely repair them and probably determine the cause of the problem.