well I did some trials with them, like enabling disabling AV/firewalls (killing services as well not just disable option) did some cookie and cache cleaning but to no help. It's not like they get disconnected or loged out. They can easily browse as long as they want in the public part of the site while being logged but when they try to get into the private parts of the site they get the same reaction as an unregisterd user.
Since I can log with their accounts easily and surf all the website without problems, I feel that there's something wrong with the way their computers behave. Anyway I should try something that crossed my mind right now, and I'll keep you posted. Voilà, discussing with someone helps to dig out new ideas