UBB.threads PHP Forum Software Community Forums UBB.threads 6 Version 6 Support Login Problem

I have PHP version 5.4.4. Sometimes when I run the website under WIN98SE and MSIE 6.0, the login page will not show the username and password window. The error message states "We encountered a problem. The reason reported was: Username/Password not found." The same site has no such problem with netscape 6.1 or MSIE 6.0 run under W2K. Does anyone else encounter this problem? Any suggest on how to fix it? Thanks!