Ran into an issue today with a client running Exchange 2010 in a Hybrid configuration with Office 365. Exchange Online users were not able to see Free/Busy information for On-Premise users. Strangely enough the below resolved the issue even though everything was already configured correctly.

From the On-Premise EMS I ran “Get-WebServicesVirtualDirectory | fl” and it showed WSSecurity being enabled but I re-ran the command as follows

Set-WebServicesVirtualDirectory -Identity “<HybridServer>\EWS (default web site)”-WSSecurityAuthentication $true

I also checked the Autodiscover IIS virtual directory but it also showed WSSecurity being enabled

Get-AutodiscoverVirtualDirectory | fl

Re-ran the same command

Set-AutodiscoverVirtualDirectory -Identity “<HybridServer>\Autodiscover (Default Web Site)” -WSSecurityAuthentication $true

Then I performed an IISRESET . After that Free/Busy information started to work properly.