Applies to Exchange 2010 SP1 RU2-5, at least.

Exchange certainly has it’s quirks, doesn’t it? It’s a great product, but it’s very complex. The more you work with it, the more you unpolished surfaces you discover.

If you have a large organization, you’ve probably created multiple Room and Equipment mailboxes. And some of your delegates might be getting this error when they try to edit calendar items directly:

What’s even stranger, is if you start up Outlook and immediately create a calendar item in the resource mailbox, it will let you do it just the once. After that, you’ll get this error. You might get a very similar bubble popping up every so often or when you access the calendar:

This happens when I edit the properties of multiple Room or Equipment mailboxes to add delegates using the Exchange Management Console. If you edit each mailbox individually, this doesn’t seem to happen. I’ve tried fixing it by removing and re-adding the delegates to the mailbox, but that doesn’t seem to work. The only way I’ve found to fix this is to add the delegates to the Freebusy Data folder with Editor permissions manually using ExFolders. The lesson: Until I find a better way to do this, do not edit the delegates for multiple resource mailboxes simultaneously.

Update: You might have to use ExFolders even if you are modifying the delegates for just one mailbox at a time.