Sorry I don't have a log or a very good bug report but over a number of characters I've had trouble with the assist command. Most recently, it was me and a shapeshifter in cobra form fighting the multi-headed dragon in arboria.

When I tried to assist it would say:
"You assist the cobra against the dragon"
They aren't here"

One other occasion I can remember with another character, we were in the arial city fighting battlement guards, I used the assist command and wound up bringing in a guard that they weren't fighting. I think I may have been hitting the second guard or something while they were hitting the first.

I've also turned back on the assist debugging to see if I can track it down without one.