"The operation failed. Cannot directly book a resource for this meeting." Outlook 2007, Exchange 2007

We recently started having some frustrating issues with resource mailboxes on our Exchange server. The issue didn't seem to affect every user, and the exact error message depended on a number of different factors. The most common is the one listed in the title: "The operation failed. Cannot directly book a resource for this meeting.", but we also saw:

'You marked " resource name " as a resource. You cannot schedule a meeting with " resource name " because you do not have the appropriate permissions for that account. Either enter the name as a required or optional attendee or talk to your administrator about giving you permission to schedule " resource name " . '

We tried SO MANY suggestions on this issue before hitting on the magic method in this thread The following steps cleared up this error for all users in all configurations:

1. Make sure you have full access permissions on the calendar (done in EMC)
2. Create an Outlook profile from the Mail control panel for the resource account
3. Open Outlook with the resource profile
4. Go to Tools > Options > Calendar Options > Resource Scheduling
5. Uncheck all options!

It seems paradoxical that un-checking these options would work. Stranger still is that the calendars continue to automatically process requests, etc. after un-checking these options. Oh well, another mystery solved!