Constantly "Forced Off" of a specific website

We have a few employees who must log into a customer web page regularly.  This web page always forces them off.  I have tested and experienced it myself.  The website will log you out even if you are clicking around the whole time.  So I assume it cannot be a time sensitive logout.  The web page will always say "Forced Off" after the web address in the box at the top of explorer when this happens.  I am just trying to figure out if there is a network/firewall change I can make to relieve this problem.  The website help desk claims there is nothing wrong with their site.  And they told us to use only explorer, which we do.

All computers in our office experiences this.  I am not certain how the site reacts to users outside of my office.  I am investigating this.

Any ideas are appreciated.  My employees spending hours on what should be done in minutes because they are unable to navigate this website and download the information they need to complete their work.