That may mean the user name is misspelled, or that the username does not include the full email address, or that the password is incorrect or that the server settings are wrong. Are you able to send messages? Can you log onto the account with your browser? If you post complete error message in English and a screenshot of your incoming server settings pane, some suggestions may come forth. Thank you

I keep getting the following prompt: Authentication required - Mozilla Thunderbird https://spica.t-online.de requires a username and password.

From the website:

"Note: When setting up your email programme, you will need a password for email programmes."

https://www.telekom.de/hilfe/apps-dienste/e-mail/posteingang-postausgang-server?samChecked=true

The message appears sporadically, sometimes every time the program starts or during synchronization, sometimes not all day. All settings are OK, otherwise it would never work.

The URL in the error doesn't refer to a mail server, but maybe it's for a CalDAV calendar that is no longer used (as the title of this topic suggests). Open the Calendar tab and remove any obsolete calendars.

https://homepagecenter.telekom.de/hilfe/konfiguration-von-e-mail-programmen-smartphones-und-tablets/kalender-und-adressbuch-mit-outlook-synchronisieren-mit-caldav-synchronizer

All CalDAV calendars are up to date.