Unable to RDP to "XXXXX" server from internal client machines and getting generic error

1) Following is the summary of the troubleshooting and the steps taken to resolve the secure channel issue.

-Reset secure channel using nltest and reboot the server , after which the issue did not reoccur.

-We also noticed the netlogon issue mentioned that the rpc was unavailable because of which it was not able to establish a secure session. Increased the number of DNS servers in the network adapter.

-Also found that the network card drivers were outdated (v4.x). the latest were around (7.x). Have advised to update the network card drivers.

-We forced the licensing server discovery over IP so that it does not depend on DNS in an event of a dns failure.

- For the access denied problem, we confirmed that it was a issue by design after not limiting the user to a single session.

we found nothing in the event viewer that states an issue with a particular component. But since we know that the issue is with the terminal server only, we have focused our troubleshooting on the terminal server itself and have tried to look for the most probable  causes of the problem.