Mail cannot move messages to Junk folder

This messages is popping up constantly in Mail: "The IMAP command “UID COPY” (to INBOX.spam) failed for the mailbox “INBOX” with server error: Some of the requested messages no longer exist (0.089 + 0.000 + 0.088 secs).?

It started happening all of a sudden on my laptop a few weeks ago, but no changes were made to settings or anything else. I suspect some iCloud issue, since this does not happen on my desktop computer, iPhone or iPad.

How can I fix this error?

Thanks!

P.S. If anyone has a resource for understanding iCloud's continual, mysterious settings problems and error messages, I'd love to know what it is.