Trying to perform many tasks results in white empty screen…

After i installed Collision Testimonials i can not perform many tasks:
– (un)installing new plugins
– modify menu structure
and more
For example if i try to update menu I get empty screen and data is not saved.
I noticed this behavior immediately upon installing Collision Testimonials. The stupid thing was that i could not uninstall it through plug-ins page because that page would not load, just be blanco.
I removed the plugin by means of FTP. Although now i can load plugins page itself, it does now work, just like wp-admin/nav-menus.php and probably other. I am glad i can modify contents of posts and pages but i really need to change the site itself.