Game Freezing / Non-responsive
The same is currently happening to me but NOT when opening/closing guild bank or calendar. It's only happening on 1 of my characters, my DK, and only when I use Frost spells. (realm: Neltharion) I've been successfully playing other characters on the same realm with the same client and had no issues. I've tried deleting cache, but it didn't help. Like the OP, I've only been able to close the game via Task Manager after it freezes.
[EDIT: I came across this thread in the search results, I didn't realize it was specific to Sargeras until after I posted. Sorry.]