adminRandomly, Office 365 users can not view the free busy information of Exchange on premise users. It was working before but just stopped for some unknown reason.
The Outlook error messages showed this error:
“An error occurred when processing the security tokens in the message.”
After much digging, I finally found used this command on both the on premise and Office 365 side:
Get-Federationtrust|Set-Federationtrust -Refreshmetadata