Forgetting to put an RDS session host back in the pool can cause overloading of the remaining hosts.
Using the following admin cmd commands (also works in PowerShell) can help. Especially drainuntilrestart as this is not an option in Server Manager.
To set a RDS Session Host not to allow new logins but revert to allowing them after a reboot.
change logon /drainuntilrestart
To check the logon availability.
change logon /query
You can also set the simple disable/enable statuses.
change logon /enable
change logon /disable
