I'm going to ask if this is for the Forum or for the Archive.
If for the archive, you may need to delete your AFF cookies, close the browser, and restart it. That may solve the issue.
If that doesn't help, then one thing to check is that you are typing the password exactly as you created it. The system is case sensitive, so if you used a capital letter anywhere, you will always need to use that capital letter.
The last thing is that you can't use the same password when you reset. If you try to reset using the same password as was previously set, it blanks the field.