When upgrading an SSL cert last week i had to run the Exchange Hybrid Configuration Wizard again. Of course when you update something like this or the AD Connect something is broken….
Before the Free/Busy was working from Microsoft 365 Exchange Online to the On-Prem environment.
After checking the relationship i saw that the sharing code was empty again.
Get-OrganizationRelationship | select TargetSharingEpr
Resulted in an empty response. To fix the Free/Busy time this needs to be filled in, in there are subdomains. If you have let’s @test.com it’s fine, but if you have @nl.test.com it goes wrong. Because it tries to reach autodiscover.nl.test.com instead of autodiscover.test.com. Even if you have some CNAME records, it takes to long.
Instead of letting autodiscover finding out what the URL is, as told in the beginning, i had this issue in the past, simply put the Hybrid servers in the TargetSharingEpr directly. So https://hybrid.test.com/ews/exchange.asmx
Set-OrganizationRelationship -TargetSharingEpr https://hybrid.test.com/ews/exchange.asmx -Identity yourmicrosoftidentity
After this, the Free/Busy will work again.