Ajonos

This is it! Repro steps: 1) Enter Combat 2) Cast Concelhaut's Parasitic Staff 3) Finish Combat 4) Save while the temporary weapon buff is still in effect 5) Load the game 6) When the temporary weapon buff wears off, your old weapon&grimoire will be gone for good and those weapon slots bugged. The issue is that saving when you're missing your weapon and grimoire due to the temporary buff will not save the items that have been removed, so when loading the data is lost for good.

This just happened to me with the custom wizard I made at the adventurer's hall. I was fighting at Magran's Fork, when for no reason my wizard was unable to cast spells. His grimoire had disappeared, and the slot wouldn't let me equip a new grimoire in it. Thankfully the autosave that was made after exiting the ruined house in that zone was before the bug occurred still had him with his grimoire. The only thing I can think of that I did that might have resulted in this during that time period was loot a grimoire from the south-east fight in the zone and teach one if its spells to my wizard with by right-clicking it when in his main inventory (unequiped). I had also saved and loaded shortly after doing that.