Ajax Error!!! error – Slider Revolution

Hello,

I recently bought enfold and installed it in my new and up to date wordpress 4.9 instance (on easyname). I wanted to install a “Slider Revolution” Slider and got the Ajax Error while trying to install it. I already edited my wp-config.php file with the suggested values:

define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );

but still receive the error. Can somebody help?

Thanks,
Christian