Password for forum

I am active on a web-based discussion forum.

Up until recently, I was automatically logged onto this forum. Then something unexplained happened, and I could not log-on. The forum moderator was accessible via e-mail, so I sent a message to the mod. The mod suggested I try "forgot your password", log-in, and permanently change my password. I did this, but not the automatic log-in will not work. The Safari / Keychain automatically tried to log-on with my "old" password. I cannot get the new password to be remembered.

How do I straighten this out?