Conditional Formatting not working anymore

Conditional Formatting not working anymore

I don't know when exactly in the last days in really happend, but Conditional Formatting as I was using quite a long time is not working on some sheets on some cells anymore. What looks strange:
  • When I hover over the range of a conditional rule, it shows "SheetName.SheetName.Range" and not "SheetName.Range"
  • When I click on "Edit" to edit a rule which is not working correctly, it does not enter the modify windows but displays an error message "Gültigen Bereich festlegen", translated "determine valid range". Deadlock. I'm not able to edit a rule.
I saw this behaviour even on an archived sheet which hasn't been touched for over 2 months. So the cause cannot be an invalid modification in the sheet itself.