How I can to debug miniBB?
Well... probably you'd need to edit index.php or bb_admin.php and put echo's right after all suspicious places to see after which string of code the invalid working starts.
But I'm not sure... may be you'd need to pay attention at your basic settings at first? Specially all $cookie* options, they must be set very properly regarding domain name and path.
Also, could you post as a guest on your board? You'd need to make all forums opened to guests for that, i.e. set $allForumsReg=FALSE;
in setup_options.php. If the posting works, and the login doesn't work, the problem may be with cookie settings.