I've had this bug in Guk. What fixed it for me was changing targets. I was going to train to zone but a group was killing spiders. I turned to kill my 3-5 frogs that I acquired running up the stairs and tabbed to the closest one and started swinging.
Later that day I stopped attacking again and I just hit the hotkey to toggle target and me twice to go back to initial mob and attacks resumed.
I'm not sure if this is the same issue, but maybe it'll help.
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