Hi Guido,
I think Karl's crystal ball is quite good. Would agree with that...
Do you have the same error message as I have?
Just to rule out. In Thunderbird there is password cashing, and I think it is independent from the account. So removing the account might leave the old password in place. You might try to clean the password via Preferences / Security / Passwords / Saved
Passwords...
There was somewhere an article that somebody changed the password and Thunderbird sent the old password and it did not work...Here it is:
https://unix.stackexchange.com/questions/164823/user-is-authenticated-but-not-connected-after-changing-my-exchange-password
"The message “User is authenticated but not connected” is due to a bug in the Exchange server's IMAP implementation. If the client presents a valid user name but an invalid password, the server accepts the login, but subsequent commands fail with the aforementioned
error message."
The IMAP interface has several problems...
The above password story is one reason why I did my own IMAP script, to be able to have full control over the communication.
So in my case this is ruled out as a root cause, but maybe you are lucky and need only to flush the cash.
Tell mi if it works...
Regards,
Robert