When accessing resource mailboxes in the Exchange 2013 EAC, you may encounter an error that may prevent you from viewing or editing room mailbox properties: 
Same goes for running
Get-CalendarProcessing <Room> in Powershell
WARNING: An unexpected error has occurred and a Watson dump is being generated : The value "already exists in the
collection.
The value" already exists in the collection.
- CategoryInfo：NotSpecified：（:) [Get-CalendarProcessing]，InvalidOperationException
- FullyQualifiedErrorId: System.InvalidOperationException, Microsoft.Exchange.Management.StoreTasks.GetCalendarProcessing
Cause: Usually this is because a disabled mailbox is listed in the room's RequestInPolicy or BookInPolicy property.
Solution: Run the following command in Exchange Powershell to clear the value. Example: Set-CalendarProcessing <Room> -BookInPolicy $null
Once done, you should be able to access the room via Powershell or EAC and add any required requesters back to the room policy