When saving in the
Location category an error message appears:
The exact wording can be slightly different. The result however is always the same: The location assignment is not saved.
This problem can be caused by creating too many automated location assignments (e.g. via a data import). The solution for this is to click the
Correction of locations button at
Administration → Systemtools → Cache / Database → Database. Afterwards the creation and editing of location assignments is possible again.