FYI, in case anyone else has this problem, the problem went away when I changed the RDP port from 3389 to a random high numbered port. I think the server was under attack as a result of all the rogue login requests (perhaps a lot of them, which were all failing), the server was unable to cope and the INTERNAL ERROR messages was produced.
We support Ukraine and condemn war. Push Russian government to act against war. Be brave, vocal and show your support to Ukraine. Follow the latest news HERE
