Mess: HTTP Error 401, 401.1 Unauthorized

Hi:

I’m getting the message:

“401.1 Unauthorized: Logon Failed

This error indicates that the credentials passed to the server do not match the credentials required to log on to the server”

I’m supplying the correct information (user and pass).
The site is up and running
My IE version is 5.50 Spanish
I’m using dial up connection

We have tried to access from several machines located in different places and sometimes it works and some others I simply can’t login to the site.
Does anybody have a clue about how this problem could be solved?