Probably their browser settings. They probably turned off Cookies and/or Session Cookies. I'd have them try another browser or reset their browser settings to the default settings and that would likely fix the problem.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.