I got alot of spell and staf missfires after update 1.4 (no beta but steam update). Not nice. Impossible to solo big groups now. It happen most when you get into a new map or if you in crt
l -right mode (croun) futher when u run around a dragon it happen 2 sometimes when u wanna heal and it miss fires with other words you cast and it do nothing (and yes full mana bar). Do others have some difficults with it (spellcasters only)

How much spell absorption do you have?

Load up an earlier save and see if you have the same issue.