When attempting to add new Remote Desktop Session Hosts (RDSH) to a session host collection, you may run into the following error message:
Unable to retrieve the session collection properties
This error can possibly be accompanied by additional messages.
When checking the logs, you apparently cannot find anything wrong with the collections: they show up correctly in Server Manager and users can still log on to the collection using existing servers.
Sometimes it can happen that the security descriptors of one of the collections get corrupted, in which case it is not possible to add a new session host, whether it's through Server Manager or through Powershell.