Resource calendar access denied

Have a resource for a conference room. When I add the calendar for this resource (Add proxy calendar - select conference room resource), I get the following message: "Access to user '(resource name)' denied"

While logged in to the resource: Tools | Options | Security | Proxy -> <All User Access> Appoinments - Read is checked
If I explicitly choose a user to grant access to, it works. The <All User Access> is not working though.

GWCHECK log shows:
- User [courthouse conference room] has granted global access rights through <All User Access>
Access granted for user <All User Access> = 64
Access granted for user <All User Access> = 17
64 Read Appointments