One account stopped working suddenly

Hi.
I have two email accounts at the same domain and on the same server. Both worked fine for a time. One has suddenly stopped working. The diagnostics say all is okay, but it won’t receive.
It shows a message saying [IMAP] Cannot open folder “/Sent”. (NO Mailbox doesn’t exist: INBOX.Sent (0.001+0.037+0.037 secs.)
This is rubbish. The Sent mail box does exist and nothing has changed in relation to the folder configuration. I did have to alter the IMAP Port number recently when for some reason the port that had been working stopped, but I don’t believe that is when this silly error message appeared.

I removed the problem account and recreated it with the same settings and it now receives mail but still shows the incorrect error message. How can I fix this please?