Problem with hosting/connecting servers!

Well, my brother has been facing this problem for 2 days. Whenever he tries to join or host a multiplayer game, the loading screen stops in the parsing game info(or something) and then, it quits back to menu, and gives a message like this; “This Steam account has been used to log in from another computer you need to log in blah blah blah”.

We installed fresh gmod installation and it didn’t help.

Advice would be very appreciated.