In my case what it worked for me was the Hosts file in the folder C:\Windows\System32\drivers\etc
I was trying to connect into a SQL server with a name that has an entry in the HOSTS file; however, the IP in the HOSTS file was unreachable - Nonexisting -
First, I connected using my SQL client using the IP address and it was no problem.
Next, I delete the entry for the SQL server name in the HOST file in the folder C:\Windows\System32\drivers\etc, saved it and again tried to connect into that SQL server using the name that I just deleted from the host file and it went thru without issues.
I hope it helps someone along the way