This will probably be the connect.ini file in c:\program files\sims\pars .net.
Just on one machine, copy the connect.ini from c:\program files\sims\sims.net to the above folder and retry.
If they are the same or this doesnt work, change the servername= line from servername\instance to ip address\instance.
If this now works, it is a dns problem, so try, on that machine putting the server in the hosts file, and reverting the connect.inin back to servername\instance. Reboot and retry. If this now works, check your DNS