DAVx⁵ has been successfully tested with RunBox.

runbox is a Norwegian Email, Web and Domain hosting platform that also offers Contacts and Calendars through CalDAV and CardDAV.

DAVx⁵ settings

Base URL:

Username: your runbox email address

Password: your account password should work, however we recommend using an app-specific password (please enable app-passwords in Settings / Security / App passwords and generate one for DAVx⁵)

Contact group method: groups are per-contact categories

Important: Make sure to use Runbox 7 and that you're in the new view where you can see Contacts and Calendars. Contacts collected from webmail can't be synchronized.

WebDAV: runbox does currently not link the files storage to WebDAV - so even if you can setup Runbox via DAVx5's WebDAV mount feature you won't see the files you uploaded in the web interface. This is probably subject to change (according to the runbox forums this is planned).