A common cause of javascript errors is web.config not being correctly configured. Review the web.config files of the two sites and carefully look at the handlers and modules sections. If you don't see anything there, review the other sections as well.
Another troubleshooting tip is to install a stand-alone installation of GSP on your site. If it works, then your broken site is missing something. Compare the two installations to find out what.
Roger Martin
Creator and Lead Developer of Gallery Server Pro