It could be the quest I'm on. Now that I think of it. I'm in waukeen's promenade hunting the red wizzards. I read the dialogue fast but I think it said something about not being able to cast spells.
I've heard (not verified myself) that fatigue gives negative towards luck, so it may affect certain aspects of spellcasting (like its effectiveness). Nothing like what you are describing though.
FYI found the problem. It seems to be the bell collar from Neera's quest that deafens people but does not go away.
I was thinking to myself yesterday that it must be some kind of silence spell whose symbol you couldn't see, but that wouldn't make sense since it gives 100% failure rate. I forgot about deaf. Try Avenger's solution and hopefully that works
Indeed.. it has currently a permanent deaf effect on a failed save rather than lasting for one minute. If you put this file in your override that should fix it temporarily.
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Or well, if it's a spell that requires a touch/ranged attack or similar to deliver its effect, then obviously fatigue will affect that.
Derp :P
The bug with the collar will be addressed.