Fatal Error with Wp Smush Pro

I've installed enda activate your plugin and begun optimize images.
I had lots of images to optimize.
After some time my WP admin page went blank an I could not access it any more.
After some debug operation (activating wp WP_DEBUG functions, manually deactivating plugin and themes, ecc.) i managed to display the error "Allowed memory size of 268435456 bytes exhausted (tried to allocate 81 bytes) in /web/htdocs/www.mam-e.it/home/wp-includes/wp-db.php on line 2416"
So I renamed wp-smush-pro folder inside Plugin directory and everything went right.
I tried reinstalling the plugin and the error went back again.
What can I do to solve the problem?
Thank you very much