since yesterday our Moodle refuses to create activities. When I try to add an activity to a course, I get an error "Unknown action" with a link to an empty doc page https://docs.moodle.org/30/en/error/moodle/unknowaction .
Database connection seems ok as text content of an existing activity can be changed or added.
Also, while the problem does seem to affect most users, some seem to be able to add activities.
What can we do to make creating activities possible again? Any idea how to proceed to further analyze the problem would also be greatly appreciated.
Moodle is 3.8.4+ (Build: 20200723)