Webservices authentication

I’m developing a website to access navision db using 2009 webservices.

Navision, SQL and website are on same Server.

In visual studio on my pc website running without problem and user can authenticate on server (from internet) but when use website version installed on server authentication fail.

The only way to authenticate is run website outside server. Credential are same.

Someone tell me what I can try?

Did you ever find a solution to this?