Broken DAV Groupware resource: workaround sought

Dear list,


I have problem with a DAV Groupware resource (Akonadi 18.04.3, 
frameworks-kio 5.50.0). It is in a broken state, giving the message (in 
Akonadiconsole Agents list):

---
There was a problem with the request. The item was not modified on the 
server.
The File or folder caldav.fastmail.com does not exist (0)
---

(caldav.fastmail.com is a Cyrus CalDAV server.)
The resource is ‘Offline, Broken’ and when restarted lands in that state 
again.

I have not found other relevant messages in Akonadiconsole, but perhaps 
there are some to be found.

Because I wasn't aware of the broken state, I have quite a few caldav 
entries in Akonadi that have not been synced with the server, i.e., that do 
not have a Remote Id. I obviously do not want to lose those.

In the past I had problems with the DAV resource before and they turned out 
to be fixable by removing some problematic entry. It might be similar now, 
but I do not know which entry, as the error information is not specific 
enough. Therefore, does anyone know how to get more specific information?

Any other suggestion also welcome. However, I can't upgrade to a newer 
version currently.


Best,

Erik