I've got to the bottom of it. I can't believe this but, it was my antivirus. The dam thing updated and decided to change the soft firewall. I decided to uninstall it just for testing, and I got straight in.
I just want to say, thank you for your attempted help.
No error running the command.
"Any chance to connect from remote via Server Manager?" - I'm not sure what you mean.
I am in via the Hyper-V and connecting using its connector.
On the Hyper-V, when I try to RDP into the server in question, I get the above screenshots.
What can I try for you?