In testing, We have been logged in on were logged in on as many as 3 PCs at once.
Your problem sounds like a cookie problem at home. While the post on the Main Forum says IE6, That is only because IE6 is the first browser that has default settings that are too low for YaBB. No matter what browser you're running, check your cookie settings. Also there are other programs out there as well that could cuase problems. CookieWall from AnalogX comes to mind.
Note that in Netscape 6.2, If you are set to "Enable cookies for the orginating website only" THAT will be your problem. Because the message board is on arcadecontrols.org, NOT arcadecontrols.com. This is to help with bandwidth.
Also one last thing. An issue using the message board is not a "BUG" until we have to rewrite code.