Mail in a loop

iMac (Retina 5K, 27-inch, 2017), macOS Mojave 10.14.6, Mail 12.4

My Mail app has gotten into a seemingly infinite loop, continuously reporting that

 The message "" could not be moved to the
 mailbox " 8. August, John_In archives —

 The IMAP command "APPEND" (to Archives/Current
 year archives/John_In archives/ 8. August, John_in
 archives) failed with server error: APPEND Contains
 Deleted flag.

Because these messages appear one right after another when I dismiss them, Mail is rendered unusable. HELP!!!

Do you have a web logon to the IMAP server? In that case, try to see if you can resolve the problem there. Else, contact your provider. This seems to be a problem on the server, not your mail app.