I had this error in my process once. Another process getting this rather frequently. Support is suggesting we might be untagging outside of the process owning lock, but it is not the case.
Typical scenario: Get Item, Do some work, Update data, Tag, Untag, Unlock/Mark - everything works fine, except Untag failing with that error. Worked fine in 6.5.1, happens on 6.10.1 - likely related to some changes, we didnt check on 7.0 yet (one changelog entry seems promising).
I could reproduce this with a simple test case on 2 concurrent running bots - one crashes within first minute. We'll add this to a ticket. I'll try to update if we get some news.