This issue has been around since we repopulated the forum several months ago. Thw two solutions that everyone seems to have made due with:
1. Clear your cookies/cache. Try logging in again. If that doesn't work for you, then...
2. Login not from
http://www.jplop.com, but from
http://forum.jpopmusic.info/ (this is how I'm able to access).
If you continue to have problems, let us know. The board definitely has a couple bugs.