I had this problem because I had entered the wrong path to the Mage.php file.
I tried erasing the plug-in and reinstalling so that I could reenter the path… but after re-installing it showed me the same error screen without giving me a chance to re-enter the path.
Instead, I had to go into phpMyAdmin and find the option with the wrong path inside wp_options. Switching it to the right path fixes the error