This is a problem with the email host's imap server, not Thunderbird. Sometimes, you can get by this by deleting the inbox (INBOX and INBOX.msf) when Thunderbird is not running. Thunderbird will rebuild it when restarted. I suggest you notify your email host of the issue if it persists.

Thank you, David, I'm suffering badly from this problem. Environment Ferdora 37 (latest GTX) TB 102.5.1 (64-bit). Service is imap incoming port 993 outgoing 465 TB folders compacted. TB Folder properties report: (after failure) Number of messages 0 Size on disk 1.7GB. Repair folder option fails with same message. TB account deleted and recreated - imap folders load perfectly - but as soon as I read a folder (any folder) problem re-occurs on a number of folders - nested and on same level as touched folder. Your comment about sometimes getting by this problem occasionally works but often it doesn't and I'm forced to delete and recreate the account. Question: What to do when deleting INBOX/INBOX.msf and restarting TB doesn't work? ISP is the largest Australian ISP and getting through to support is hard work - and they will tell you they don't support TB on their service. No problem with Android K9 mail but Windows 11/TB environment fails with similar results. Look forward to hearing from you.

I wish I had good advice for you, but I do not. There are some people on this forum with far more technical knowledge on servers than I. However, I do agree that it is difficult to get an email provider to fix a problem that does not appear within their narrow support window.