Hello Roberto,
Thanks for writing in!
X theme should be upgraded to Pro theme and not the other way around. I have checked your licenses and I am seeing both the X and Pro license codes.

Meanwhile, a blank screen could mean a server settings is not correct, fatal error or PHP memory limit exhaustion. Can you please go to X > Status, copy the results and paste it in your next reply so we can check your server settings? Meanwhile, we recommend that you increase your memory allocated to PHP. To increase it, please edit your wp-config.php file and insert these lines:
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
You can add it before the line
/*That's all, stop editing! Happy Blogging. */
We would loved to know if this has work for you. Thank you.