Blank pages could be an indication of PHP errors. What PHP version are you running (ask your hosting company this if not sure).
Suggest you log a ticket with Appnitro, and your hosting company helpdesk - ask about turning on Error Reporting so yo can see on screen any PHP errors that might be able to point to the cause.
Not sure why your MF v9.0 works but MF v11.0 doesn't. Did you upload the 11.0 files as "Auto" mode in your FTP? Can you check in FTP, the file permissions are correct?
(These sort of things might be checked by Appnitro anyway)