While working a case where an admin was having issues migrating users to the cloud we came across a fairly common error.
Cannot open mailbox
/o=ORG/ou=Exchange Administrative Group(FYDIBOHF23SPDLT)/cn=Configuration/cn=Servers/cn=E2K13/cn=Microsoft System Attendant
Now normally we can correct this behavior by modifying the HomeMDB of the system attendant to rehome it to a different database, or by mounting the database where the system attendant mailbox is homed (if it is dismounted). Unfortunately, in this specific scenario rehoming the system attendant mailbox did not correct the behavior. Now in this current configuration we were migrating users from 2007 to the cloud and the 2013 server was being used temporarily.
Another symptom we noticed is that when attempting to access room details in Exchange 2013 we were receiving this same message, not being able to connect to the system attendant mailbox. We were able to narrow down the issue to the LegacyExchangeDN of the system attendant mailboxes on 2013 which
was causing lookup issues for the 2013 system attendant mailbox. There was on slight modification to their Legacy DN's.
/o=ORG/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Configuration/cn=Servers/cn=E2K13/cn=Microsoft System Attendant
/o=ORG/cn=Administrative Groups/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Configuration/cn=Servers/cn=E2K13/cn=Microsoft System Attendant
We were able to discover this by using LDP on the server to grab the attributes of the 2013 System Attendant mailbox and compare it to output from my lab. By just adding the Administrative Groups to the LegacyExchangeDN attribute it caused issues with the lookup of the System attendant mailbox in Exchange 2013 which caused several issues. This particular issue is not common because this particular attribute (to my knowledge) can only be manually modified in this fashion, however if you're having some odd issues moving mailboxes, or gathering specifics on Room mailboxes it's one thing to check to make sure you don't overlook anything.