Go to Settings -> Applications -> Manage applications -> N9 QTweak -> Uninstall. After this go to /opt folder with WinSCP or FileBox as ROOT and check if you see folder named "N9QTweak". If yes, delete this folder. Go to Documents folder and if you see the PDF instruction (N9_QuickTweak_en_v6.5.pdf and N9_QuickTweak_ru_v6.5.pdf), delete them. Reboot your phone and try to install again by double click on the .deb file.
1. install previous version that worked for you 2. Reboot your phone 3. Go to Settings -> Applications -> Manage applications -> N9 QTweak -> Uninstall 4. install new version by double click on the .deb file