The log-in failure may be due to a session time-out. Try closing PageBuilder, then clicking the "Sign Out" link at the top of your GeoCities Control Panel. Sign back in with your Yahoo! ID and password, then launch PageBuilder again. This should fix the issue.

Another possibility is that you have a program installed on your computer that blocks cookies. If so, disable this program and make sure cookies are enabled when you try to launch PageBuilder.