Many of my mailboxes on the IMAP server just emptied. They were still there in my Ubuntu '.thunderbird' ImapMail directory. I have created some replacement mailboxes in 'Local Folders' and copied mbox files from the ImapMail area to the Local Folder area, removing the .msf files just created by Thunderbird. I then used the Properties->Repair Folder on each of the the new Local and filled-in-the-background mailboxes. Many worked fine and I was able to repopulate the IMAP server mailboxes using drag in Thunderbird. Some, however, were mis-parsed, producing 'From-less' and Subject-less messages containing part messages. Thunderbird seems unable to reliably parse mbox files created by itself. I note that they are not strict mbox format, having a '-' instead of an envelope address on the delimiting '^From ' line. (I am a retired Unix SysAdmin, so used to the command line)