Process Microsoft.Exchange.Directory.TopologyService.exe (PID=2092) Forest XXX. Topology discovery failed, error details
No Minimal Required Number of Suitable Directory Servers Found in Forest XXX Site XXX and connected Sites..
This issue appeared when we added a tree domain to the forest. For some reason 2 AD controllers weren't enough.
As administrator edit Microsoft.Exchange.Directory.TopologyService.exe.config file in the C:\Program Files\Microsoft\Exchange Server\V15\Bin directory. Add this attribute in the Topology section: MinSuitableServer = "1". I also changed MinPercentageOfHealthyDC to 10. Not sure if the last one is required. Reboot or restart the services and the error should be resolved.