Have the same bug, and it's a simple bug )
When some1 cast a spell that alter his weapon - such as druid flame sword or wizzies balls of acid - this crap happening sometimes after spell duration is over. Your weapon simply disappear, and in this Aloth's bug - his grimmoir too. In fact Aloth will attack only with fist after that and unable to cast any spell =(
Fix this ASAP plz, this bug ruin a game =(