If you running Hybrid Environment or just trying to share free/busy information between your on-premise Exchange Organization and Office 365 you may experience that free/busy information for some mailboxes which were recently moved to O365 is not available. Please note that problem does not affect free/busy from on-premise to O365, if you cannot get free/busy for on-premise mailboxes – it must be another issue.
In addition you may see error similar to following in Application Event Log of your Hybrid server:
Event 4002, MSExchange Availability Process 4192: ProxyWebRequest FederatedCrossForest from S-0-0-00
-0000000000-0000000000-000000000-0000 to https://server_name.outl
ook.com/EWS/Exchange.asmx/WSSecurity failed. Caller SIDs: WSSecur
ity. The exception returned is Microsoft.Exchange.InfoWorker.Comm
rvices.Protocols.SoapException: An internal server error occurred
. The operation failed.