I've completely removed my FastMail account (both IMAP email and Calendars) from Thunderbird and re-added them after closing and restarting Thunderbird, but keep getting the aforementioned authentication dialog. All saved userids and passwords relating to FastMail have been removed in Thunderbird's password manager.https://carddav-dnnnnnn.fastmail.com is requesting your username and password. The site says "carddav-dnnnnnn.fastmail.com"
Is it FastMail that's providing that username to use to connect to my FastMail calendar? How would I find out where that username is coming from?
[edited to remove unique identifiers]