[horde] "Admin access not enabled" in Kronolith

[Michael]

Hi,
when trying to add or modify any calendar or task list in Kronolith the
users are getting the alert "Admin access not enabled".

A few weeks ago it used to work fine.
What does the alert "Admin access not enabled" mean?
What do I need to change to get it working again?
The rest of Horde is working fine.

Calendar (kronolith) 4.2.4
Horde (horde)        5.2.3
Mail (imp)           6.2.6

Thanks,
Michael