@joosuak
Hello, I fixed it on my site by changing the code of the function into :
// fixes the plugins_url
function domain_mapping_plugins_uri( $full_url, $path=NULL, $plugin=NULL ) {
$wp_content_dir_exploded = explode(DIRECTORY_SEPARATOR, WP_CONTENT_DIR);
$wp_content_foldername = $wp_content_dir_exploded[count($wp_content_dir_exploded) - 1 ];
return get_option( 'siteurl' ) . substr( $full_url, stripos( $full_url, $wp_content_foldername ) - 1 );
}
I have defined WP_CONTENT_DIR in my wp-config.php but it is also defined in wp-load.php so it whould work for you.