Is this another with outlook.com?

Not outlook.com but it is an account that is hosted on outlook.office365.com.

Is there a known problem with TB + Microsoft hosting?

This account works fine on my iOS devices. Only TB has the problem.

What's the solution?

At this point we have no solution. We don't understand it really. But the symptoms appear to exactly match those outlook users had with the same site after the November patches were applied.

Are you technical enough to log the connection so we might see where the error occurs?

See https://wiki.mozilla.org/MailNews:Logging

Note Bug 1241926 is being used to manage this issue. So any breaking news will be there first.

The November patches you're referring to - are those in TB, or the Outlook server?

Please keep in mind all of this worked perfectly until yesterday. Something changed yesterday while I was using TB. Could something be corrupted in TB? If so, how can I fix that?

I don't know how to log the connection but if you point me to a description I may be able to do it.

I already pointed you to the description of how to log a connection. I think we are seeing a progression of some patch rolled out in Microsoft's data centre, one server at a time getting the patch and another bunch of folk with folder issues. I really do not think it is Thunderbird at all..

It would appear that Microsoft released a bad update to their office365 / outlook.com servers which broke IMAP folders. Microsoft are working on the issue and promise to remediate the situation. For a workaround, please see my blog post. Unfortunately other an that there is nothing more we can offer until Microsoft fix their servers,

We are tracking this issue in Bug 1241926.

Thank you Matt. This is probably good advice. And I look forward to Microsoft's update.

Note however that this change didn't make any difference with the problem on my system. What did make all the difference, as far as I can tell, was changing the maximum number of server connections to cache to 1 (from 5) in TB - with that change it works fine. This was suggested by my university's IT department, who recognized an incompatibility between TB and the outlook.office365.com server. Perhaps that's a Microsoft problem, I don't know. It's great to have TB working properly again.

FWIW, all this time the same account and server have worked fine on iOS mail.

In your case it might be ios mail that is the root cause. ios mail uses multiple connections as well, but Apple provides no option to limit their use So IOS and OSX devices tend to use all available IMAP connections until the pool on the server is exhausted. Other apps never even get a look-in..

Further update: My problem was resolved for about a day after I (a) unchecked "Show only subscribed folders" and (b) set "Maximum number of server connections to 1" (from 5). However, again today suddenly TB stopped showing any folders other than Inbox and Trash for my IMAP account, while I was using it. I checked those two items above and it had, on its own, changed setting (a) above to be checked, when I had left it unchecked. Unchecking it did not solve the problem. But I closed and restarted TB and now it seems to work, and now both items above are as I had set them. Apparently TB is changing some settings on its own.

If Thunderbird had not completely shut down before you restarted, due to crash, hang or computer restart while running, it is possible for these settings to not persist.

> However, again today suddenly TB stopped showing any folders other than Inbox and Trash for my IMAP account, while I was using it.

This of course is still the outlook problem, combined with the prefs workaround not being in force.

Just had this type of problem this morning, in my case it is an eMail address with @live.de domain that also belongs to outlook.com The eMail folder-structure all of a sudden disappeared, and re-appeared when I clicked the subscribe/refresh button (several times). However interesting --- Thunderbird seemed to have deleted all local contents of the formerly downloaded imap-folders. Thunderbird had to re-download everything all over again. No problem it worked and nothing got lost. But it seems that if something in the folder structure list is broken server side (aparantly it is MS fault) then Thunderbird immediately gets rid of the folder-tree and deletes the local copies of the imap-folder structure. Anyhow, all is well and presumably this is not a thunderbird malfunction.