I received an error on our QNAP NAS which runs WordPress when trying to update to version 5.0.1 via FTP:
“Unable to locate WordPress root directory”
I haven’t had a chance to get down to the … ahem … root … cause but it looks like it is something to do with permissions. The following code added to the bottom of the wp-config.php file did provide a workaround and permit the upgrade:
if(is_admin()) {
add_filter('filesystem_method', create_function('$a', 'return "direct";' ));
define( 'FS_CHMOD_DIR', 0751 );
}
Source: StackExchange