Windows Server 2016 - Can't RDP with one specific user

What worked for me finally was to logon as a different admin user and start Task Manager, switch to the Users tab, right-click the problem user and select "Connect".

Just had the same issue. Interesting that it was only me. Here's my fix. Since the message said RDP service is busy. But there were no sessions open. I went backdoor into the server via "Computer Management" and restarted the RDP service. Worked like a charm!!!

I had this exact problem. When I got in with the one account that could connect, I found in the Users tab of Task Manager that there were four sessions with null usernames. When I ended the Windows Login Application task for each, the null entries disappeared and I was able to log in via RDS normally. It did not require a restart.