Ran into a strange issue recently where on-premises users could not see the free/busy information for test users I had migrated to Exchange Online. Exchange Online, on the other hand, had no problem seeing the free/busy of the on-premises users. I had just run the hybrid configuration wizard and it completed without incident. The environment had not been previously enabled for the Microsoft Federation Gateway so I let the wizard take care of that step as well.
When we tested the trust with the federation gateway we received the following error on Step 5 of 6: Requesting delegation token.
C:\> Test-FederationTrust -UserIdentity email@example.com <...edited...> RunspaceId : e4e42cab-62f4-4d70-be15-69143b273823 Id : TokenRequest Type : Error Message : Failed to request delegation token. Error. Attempted to get delegation token, but token came back as null.
The token coming back as null seemed to be the key here. Unfortunately, the web seemed to lack any real way to fix the existing trust. We resolved to delete and recreate the trust with the federation gateway.
Recreating the Federation Trust
To delete the federation trust navigate to the Organization > Sharing tabs in the Exchange Admin Center. Under the section titled Federation Trust click the Remove button. Click Yes to confirm.