Two users were updating the same map (but different elements).
After update (or maybe this just coincided), the map does not open anymore and we get error message:
at the bottom of the screen.
Error is shown all the time and map is not usable. Some part of it is visible from the admin screen, but from: Monitoring -> maps there is nothing visible, and some more error messages:
I tried to reproduce with another map on another instance - could not reproduce therefore unable to say if it was related to two user simultaneous editing.