To solve this problem, enter the following code inside your .htaccess file:
<files xmlrpc.php>
SecFilterInheritance Off
</files>
This code will instruct the Apache server to exclude xmlrpc.php file from the security filter check.
note: Don’t forget that the line ending of the .htaccess file is of type Unix (LF) before submitting it to your server, if it is a *nix based server. Otherwise, you are going to get Server Error 500.