Solution:
WP Super Cache puts the below line in your wp-config.php file. Remove the line from your wp-config.php file and reinstall and reactivate the plugin.
define( ‘WPCACHEHOME’, ‘/home/USERNAME/public_html/wp/wp-content/plugins/wp-super-cache/’ ); //Added by WP-Cache Manager
I encountered the same problem after moving my WordPress directory to the root of my site. If you get the problem and can’t deactivate the erroring plugin, go to your /wp-content/plugins/wp-super-cache and remove or rename the directory. This will manually deactivate this so you can install a fresh instance.