Best answer by NinjaGeek
Updated on 17/09/20: make sure to use your forum username rather then email address when logging in. See below for more info on this and advice on deleting browser history and stored password
Sounds like a caching issue. I would recommend you clear your browser history and stored passwords and try again. Make sure you are using your username you set up and not your email address too when you try and login.
One way to find out is if you are using Google Chrome as your web browser. Press Ctrl Shift N to open an Incognito window and try logging into the forum this way. If it accepts your password first time, then this is the issue.
Let me know if this works!