Thanks, yes, we have a firewall and yes there was a problem with the rules. We fill in sheets with the FROM and TO IPadresses and the ports that we need. The UDP 1434 was on there (also for the physical adresses) but the engineer that was working with it forgot or missed that entry....

The question that comes to mind with that is why I could connect to the database if didn't put the named instance in the connection string? Does it communicate differently? And what tool can I use best to see if the TCP and UDP ports of a machine are open?