It is very difficult to give a precise advice, because it all appears some misconfiguration or the wrong customization on your end, which you are responsible for. You may start just from the default, non-customized miniBB and check all this gradually, if it will work in default version - you will need to re-check the changes you've applied one by one.
Empty pages may be a result of some error occuring during execution, but you do not see it on the screen because you may have error_reporting = NONE or something similar in php.ini. You need to set it to E_ALL to see all erors on the screen, which may give you paths to solving the proble(s).
I am not sure how you did "fitting" of forums pages to your layout; basically all files you have to modify are templates/main_header.html , templates/main_footer.html and bb_default_style.css. If you go beyond of that, your experience may not be just enough for doing more complicated thing.
As about synchronizing membership, with miniBB it's quite possible and I did it numerous times for different systems; but this is for sure the work for educated coder and not for newbies:miniBB Forum Integration into 3rd party membership / login authorization system