Thanks very much for looking into our problem. We used Sync Jobs (about 80, one for each account) to bring in the emails. The problematic folder did not exist on the original server. It’s not a client problem (I think) because we asked for a 24 hour outage and so no one except myself has used the accounts, even with Webmail.
We have tried SO MANY things, but it always comes back and it is certain that the server is recreating it every time. I don’t know if it’s SOGo or mailcow, but it is happening on every account and I cannot explain it. We’ve also deactivated all the sync jobs, so I don’t think it’s the sync job code either, but perhaps I’m wrong.
I guess we could reparameter every Thunderbird account to use the “Archive” and not “Archives” folder, and then drag and drop all the subfolders, but that’s frankly a huge hassle.