My help desk gave me a shout today saying that a particular user could not launch an RDS app from their computer.  Naturally I checked all the basics with test account and all was operating as expected so I knew it had to be something with the client machine.

I did a bunch of more basic troubleshooting (I didn’t have a lot of faith in this particular technician) and then started digging through the logs on the RDS Gateway box until I found the following little bugger (Event 4625):

Naturally this doesn’t tell me anything, but with a little Google-Fu I came up with this source.  I changed the LmCompatibilityLevel value to 3 or higher as directed, gave the machine a reboot, and wha-lah it worked.  The value can be found at HKLM\SYSTEM\CurrentControlSet\Control\Lsa.