I have the same error, I already use wordfence last week but today put a warning.
I have to delete the pluging at the cpanel. But now I have more errors.
Warning: file_exists() expects parameter 1 to be a valid path, string given in public_html/wp-includes/theme.php on line 400
Warning: file_exists() expects parameter 1 to be a valid path, string given in public_html/wp-includes/theme.php on line 404
Warning: Cannot modify header information – headers already sent by (output started at public_html/wp-includes/theme.php:400) in public_html/wp-content/plugins/wp-super-cache/wp-cache-phase2.php on line 1338