An Area Display or Reception Display does not show any information at all - when showing one of these Displays it shows a black screen.
One or more mailboxes for a room does not work - due to missing/invalid settings,
Please go to Administrator->Settings->Rooms to locate the mailbox with missing/invalid settings
HOW TO SOLVE
Either solve the missing/invalid settings for the affected rooms, or remove the affected room(s) from the Area and Reception displays that doesn't work