There was a problem retrieving the XML data

Some users can't use the system becouse after they login some errors messages appears.

"There was a problem retrieving the XML data" with different details like  "Access Forbidden" or "Proxy error ( The specified network name is no longer available )"
"uknown" or
Can you say us the origin of  these errors?  

It's very important for us to solve this problem.
Kind Regards
MG

ps. the firewall on my server is disabled. And I use an ADSL access to connect my trial server to the internet.

The error may be caused because IIS on your server can limit the number of concurrent user connections to 10. It can also keep the
connections made (through Web requests) alive for 15 minutes by default.
This behaviour is designed to conserve/reuse resources, but it
seems that every Web request is regarded as a connection by a different
user - so you soon run out of resources.

Please take a look at the following KB articles explaining this and the solution:
Error Message: HTTP 403.9 - Access Forbidden: Too many users are connected
How to optimize Web server performance in Windows 2000

The keep-alives can be disabled through IIS admin interface. I hope this resolves your problem.