When backend protection is enabled uses the full url to access backend, so it's important to replicate it to access the backend. This means that putting www or https (if backend protection was created when ssl was not enabled) matters.
If you're not sure of the full url just edit your .htacess file (into root folder) and look for the #begin backend protection text. A couple of lines behind it you will find it.
If this doesn't work please send me administrative credentials to access your backend and also for cPanel (or ftp). Use the email from which you receive forum notifications.