I've had this on the exchange 2010 forum and had no answer yet.

I've set up two room lists in a large organisation (50K users) running pure Exchange 2010 across a huge cas array and multiple mailbox servers. I've added rooms to both lists and also checked that the rooms are set as the correct resource type. However the room finder is empty. The problem occurs in both cached and online mode. Both groups were set up months ago (so not an OAB update problem) and are set as Room lists correctly. Also I can see the groups if I select them from the GAL. This issue affects all users.

I enabled Outlook logging and get two errors when using the scheduling assistant:
2013/05/22 09:27:21.403: CRoomsCTP::FOnPutAppointmentItem(): HrSetASUrl failed. 0x80004005

2013/05/22 09:27:21.840: CRoomsCTP::HrGetRoomListAsync(): HrGetRoomLists failed. 0x8000ffff.

GAL is updating fine as is OAB. I have googled extensively and searched similar threads here but have not found an answer.

I have also checked the worktimes for the rooms in each group and they are set to 8.00am to 5.00pm so its not the outlook hotfix issue.

I've now run a test on our lab and Room Lists work fine in that environment- both cached and online. I've turned on OAB logging and no errors are reported that are pertinent.

I've created a brand new room list and room on the affected environment with still no luck.

solved by Microsoft premium support. We use Outlook Anywhere (not MAPI) for connectivity in preparation for Exchange 2013. In this case you must set Configure External Client Access Domain. WE don't allow external access so we set this the same as our internal URL and it fixed the issue