Exchange 2013 user getting "Cannot open Calendar folder for user" for mailbox on Exchange 2007

Hi All,

I have a coexistence scenario with 1 2007 server and 1 2013 CU9 server. I've moved some mailboxes over to 2013 and while most things seem to work fine, I've got an issue with room resource delegates.

I have a room resource mailbox, we'll call it Room1 that is still on the 2007 server. It has a ResourceDelegate set as UserA because this user is in charge of accepting or declining requests to use the room. I've moved UserA's mailbox over to 2013 and now when they get a request to use the room, the forwarded invite shows "Cannot open Calendar folder for user Room1. Microsoft Exchange is not available."

Can someone help me fix this? Here is some output from Get-OutlookAnywhere

ExternalHostname                   : mail.mydomain.com
InternalHostname                   :
ExternalClientAuthenticationMethod : Ntlm
InternalClientAuthenticationMethod : Ntlm
IISAuthenticationMethods           : {Ntlm}
ExchangeVersion                    : 0.1 (8.0.535.0)

ExternalHostname                   : mail.mydomain.com
InternalHostname                   : mail.mydomain.com
ExternalClientAuthenticationMethod : Ntlm
InternalClientAuthenticationMethod : Ntlm
IISAuthenticationMethods           : {Ntlm}
ExchangeVersion                    : 0.20 (15.0.0.0)