Error messages are not always 100% accurate so instead of focusing on the exact message I would suggest double checking all possible, easy, failure points
1. in the instance to which you are trying to connect (let's call that computer A), go to the Server tab (next to file and tools) and then the User/Logon tab and verify that that the setting 'Only allow local connections' is NOT checked
2. Check that you are using the right port by going to Server - Settings (like in step 1) then Main Settings and check the port and the firewall
3. Unlikely, but depending on the security on your network, is it possible that Computer A cannot talk to the computer that is trying to connect (let's call that Computer 😎. One quick way to sort that out without help from an IT security person at your company might be to; check or setup a quick network share, ping computer A from computer B or maybe run a traceroute from B to A
4. The user that is trying to connect, have them log in locally on Computer A first before you try to connect them remotely
5. Is the user a local user OR an AD user? If they are an AD user, is it possible they are locked out?
6. If nothing else try to logon (from computer 😎 as the default "Admin no password" user
7. maybe double check your IP or try to use the name of computer A instead of IP
I will try to think of some others. I know the message indicates a connection issue but it could certainly be a user issue...I think that is more likely but it depends on your network