Room mailbox availability error “An unexpected error occurred and your request couldn't be handled”

When booking room mailbox for meeting which is on exchange server 2003, getting error “An unexpected error occurred and your request couldn’t be handled”.  This user has been moved on exchange server 2010 and when user tries to book a room for meeting he is getting the error in Room Mailbox Availability status.

Room mailbox booking for meeting
Availability error

Ran the command “Set-Mailbox User1 –ApplyMandatoryProperties” and checked the issue but it didn’t worked. Checked the properties of Room Mailbox for permission and if user is part this room Mailbox, and permission and user were intact.
I decided to move this Room Mailbox to exchange server 2010. Moved the Room Mailbox. When you move the room mailbox from one server to another server or database, it will be moved as user mailbox but not as Room Mailbox. You will have to convert the user mailbox to room mailbox.

Set-Mailbox COConf -Type Room
After converting the user mailbox to room mailbox, checked the issue but still it wasn’t working. There is another setting which need to be changed and it can be done using EMC or EMS.
Go to properties of Room Mailbox…Resource General TAB and found that “Enable the Resource Booking Attendant” is not checked.
Room mailbox properties “Resource General”

Make sure “Enable the Resource Booking Attendant” check box is checked!

Enable the Resource Booking Attendant

You can make this change by running the command “Set-CalendarProcessing “COconf” -AutomateProcessing AutoAccept”. After making this change, checked the issue and how I can see the availability of Room Mailbox.

Room availability working

Now, it started working for the user’s on exchange server 2010, but what about the users on exchange server 2003? Checked the users of exchange server 2003 and it’s working fine.