DAV groupware resource broken/offline with message "The file or folder localhost does not exist"

Hello,
I am using a setup to access an Exchange calendar via local DavMail via Akonadi. The calendar synchronization worked for several months but stopped some time ago. The calendar resource and all its sub-calendars are marked as “offline” in KOrganizer. When trying to refresh the calendars, KOrganizer asks whether to switch the calendar to “online”. If approved, it tries to activate it, but flips back to “offline” after a second. No further error message. Old calendar entries stay visible, presumably because they are cached.

DavMail itself is still working, as I can send and receive mails via its IMAP/SMTP interface. I can also synchronize calendars independently of Akonadi via vdirsyncer, using the same configuration values (username, password, URL).
In the DAV resource’s settings in Akonadi, I can fetch a list of discoverable collections, so communication from Akonadi to Exchange happens at some point (i.e. credentials are valid).

To investigate, I used Akonadi Console. In the “Agents” tab, the details on the broken DAV groupware resource show the following information:
Status: Offline, Broken
Status Message: The was a problem with the request. The item was not modified on the server. The file or folder localhost does not exist. (0).
Capabilities: Resource, FreeBusyProvider
MimeTypes: text/calendar [MORE CUT AWAY]

Any ideas what the problem is?