I had a tricky issue with a server coming up with errors when trying to add it to Server manager > All Servers
It came up with target inaccessable and when I tried to remote to the server I received SSL certificate errors. I inially though the error was on the server I was trying to get to join server manager but after hours I still was unable to make it work.
I had however changed the IP address on the server the previous day. This should have given me a clue earlier.
The server running Server Manager was a DC, so I thought that this might be where the error was.
Essentially the DNS settings had a stale (previous IP) of the server hard coded, and it had not updated after my ip address change.
I changed the IP in the DNS settings for the server to the correct IP Address and then flushed the DNS which allowed the connection and remote management.