The character makes the casting animation, and the spell sound goes off, and I think it costs SP too, but no spell effect happens at all.

Seems to happen sporadically: if it stops working, it will not work for a few minutes, then later it will work fine, then stop working again.

Doesn't seem to matter what I have targetted. I've tried no-target, target floor, target mob, target self, all randomly not working.