When I configure a Runbox account in Thunderbird using the new UI (Account Hub), it unsuccessfully tries to discover the calDAV and cardDAV endpoints for calendars and address books.
Tested on Thunderbird v146
I have 2fa enabled so it might have something to do with that. The documentation doesn’t mention app passwords for dav access.
To confirm my theory I followed the documentation step by step. It neither works with my primary account password (which makes sense as that would circumvent 2fa) nor with an app password. Can somebody confirm that?