Removeall give "invalid or nonexistent document" error

I have an agent that has suddenly started giving the error “Notes Error: Invalid or Nonexistent Document” when I do a removeall for a document collection. The collection has entries - I suspect some form of corruption but am not sure how to resolve. I made a copy of the NSF file via “applications/new copy” and copied design/acl only with no luck.

Any suggestions?

Thanks