SUMMARY
Everything worked fine until yesterday's update. After the update, Akonadi can no longer find the CalDav server to an Open Xchange (OX) account. The server can be found by the OX web interface and using the Exchange account settings in Android. So, the server does exist and updates outside of Akonadi.

STEPS TO REPRODUCE
1. Open Kontact or KOrganizer; the calendar appears as it was 2 days ago.
2. Open the Settings and go to Calendars.
3. Select the CalDav calendar listing, click on Restart. 

OBSERVED RESULT
Message comes back "There was a problem with the request. The item was not modified on the server. The file or folder dav.siteprotect.com does not exist. (0)"

EXPECTED RESULT
Update the calendar

SOFTWARE/OS VERSIONS

Linux/KDE Plasma: OpenSuse Tumbleweed 20201106
Linux Kernel: 5.9.1-1
KDE Plasma Version: 5.20.1
KDE Frameworks Version: 5.75.0-1
Qt Version: 5.15.1

ADDITIONAL INFORMATION
There are calendar updates since the last successful sync. The sync worked fine until OpenSUSE update yesterday, which updated the Akonadi system. If I try to edit the calendar account and Modify the CalDav server in the General Configuration dialog and click on Fetch, Akonadi finds all my calendars assigned to that server. However, when I click OK and exit General Configuration and Restart, I receive the error message noted above.