Hello @Darijo,
Thanks for writing in! An error 500 could be caused by a fatal error. A fatal error which usually related to incorrect php code or PHP memory exhaustion. Could your please enable the debug mode? You can do this by opening wp-config.php and adding
define('WP_DEBUG', true);
/* That's all, stop editing! Happy blogging. */
When you revisit the page, you should have some error output describing the issue in more detail.
And we also 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.