Ok, now that is a bit odd because there are no validators on "assign issue". That makes me suspect that you might be doing assign in the workflow, or that it's something else, so can you do some quick extra tests for us?
That's not the assign issue action, that's in the workflow, in a transtion labelled "assign issue" You've got a broken validator on the transition. Probably someone removed a plugin that provided the validator. You should read your log to find oout what the validator was, then you can put it back, and re-edit the workflow to remove it if that's what you need to do.
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