Thunderbird requests a password only when the email provider sends a request for it. Does the password work until the next restart? More info on this is needed before anyone can give suggestions

I have a similar issue, and I believe I've seen this happen for a long time (like, years). This seems to happen every so often when changing passwords for IMAP and/or CalDAV accounts. My working theory is, when the password changes on the server end, Thunderbird has (correctly) decided that the stored password needs updating, remembers that, and after that erroneously keeps asking for it every time it starts up, even though the password has already been correctly updated in the password manager. Deleting and recreating the password in the password manager does not seem to make a difference. Removing the whole account and recreating its settings in Thunderbird does seem to help, but it's a bit silly to need to do this for a password change. Suggestions on how to debug this? What is the mechanism for remembering that a password needs updating?

Please remember that Thunderbird is not asking for the password. It serves only as an agent for the email provider, passing on the request that is received from provider. My suggestion to stop the refusal of password is to click settings>privacy&security, click saved passwords, then show passwords, and then delete the rows associated with the account. That stops the refusal of password because now there isn't one. You should now receive a prompt for the right password. At least, that's the norm.

As I said, I've repeatedly removed and added the password back. And no, as I also said, this does not prevent the password from being asked again, even though I can verify it has been saved correctly.

As I also also said, this is something that has been with Thunderbird for a long time: occasionally passwords seem to get stuck in the "not valid" state, and will get re-requested every time Thunderbird starts.

Again, any suggestions on how to debug this? What is the mechanism for remembering that a password needs updating?

Debugging your situation would probably start by removing all password info by going to settings>privacy&security and removing the rows for the account and then setting fresh. This depends also on the email provider. For example, with gmail, submitting the wrong stored password does not result in a request for a new one due to OAUTH2 settings. In this process, thunderbird is stupid, in that it just replies to whatever the email provider requests, making no decisions on its own.

In this case, the mail provider serves IMAP (993), CalDAV/CardDAV (443) and SMTP/submission (587), all with password authentication. Upon some thinking and looking at the server-side logs (since I have access to them), it was the CalDAV account that was keeping getting re-requested. However, according to the logs, nothing seems to be happening yet with that part of the server when the prompt is displayed, so it's something internal to Thunderbird.

I finally figured out what was wrong in my settings, by looking at the calendar properties:

Username: username@server.domain Location: https://server.domain/dav/username/Calendar/

But when prompted, I had been just entering username (without @server.domain) and the password — the server happens to accept this, and this gets stored into the password store, and everything works fine for that Thunderbird session. Now, the next time Thunderbird is started up, it again looks for username@server.domain in the password store, and prompts you for it because exactly this value hadn't been stored there.

When I corrected the Calendar properties Username to not have the @server.domain part in it, Thunderbird used the matching username/password values already stored in the password store, and stopped prompting me for it at startup.

Morale of the story: always know what each setting signifies (it'd be nice to have better info popups). I might even log a bug about this: the username/password prompt really should come pre-filled with the complete username Thunderbird is expecting to use, as this should match what is in the calendar properties (or you'll get repeated prompts, like I did).

Thanks for sharing that. I have also noticed that the account setup window just uses the left part of email address when setting default user name.

Created a bug report for this issue https://bugzilla.mozilla.org/show_bug.cgi?id=2004690