Akonadi won't play with webdav

[Borden <borden_c@tutanota.com>]

It seems recently that Akonadi won't play with my CalDav and CardDav server. The server works because DAVx5 and the web interface work. On Debian bullseye, fully up-to-date.

I've run the logger in Akonadi Konsole, restarted Akonadi, cleared the caches, checked the settings, all to no avail. My address book and calendar are both gone from Kontact.

The logger shows nothing of interest to me, so either I don't know what to look for, or Akonadi also doesn't provide useful  information.

Questions:
1) Does anyone else have the same problem?
2) Can anyone suggest a troubleshooting procedure to figure out why it's not working?

With thanks,