i got kinda the same problem aswell (it's in coop mode too i din't try it solo).
No problem for summoning but i have the polymorph problems :s
Maybe can help: i had the beug in my "main" (custom) carachter and on loshe. But now some spell (that i use for several levels) are working.
My guess is when you learn a polymorph (or maybe summoning) skill with a character it's kinda bbugging during the whole level you learnt it? (it's rather strange i agrea). Cause i have no more problemme with my "old" polymorph's skills.
Hope will be helping finding and fixing the pb ^^
Lich,
