|Check the settings for the ISPs DNS server at the router or server.|
If you are able to ping and traceroute on both lines, whats about the name resolution on a traceroute?
I guess it only functions, when you've switched back, right?
The new ISP uses other DNS servers, you may have to configure at the router or server.
The original poster should always write the last response !!!
Let us know, if the problem is solved !!!