Elementor - Fatal error: Unknown: Cannot use output buffering in output buffering display handlers in Unknown on line 0

Error message
Fatal error: Unknown: Cannot use output buffering in output buffering display handlers in Unknown on line 0
Cause

The  memory_limit value is reached on your server. To confirm that this is the problem, write this code inside your wp-config.php file right under the <?php part:

error_reporting(E_ALL);
ini_set('display_errors', 1);<br>ini_set("log_errors", 1);
ini_set("error_log", dirname(__FILE__)."/php-error.log");

and go to your page once again with this error. You should find a php-error.log file next to your wp-config.php file after that and this should tell you the cause of the problem. If it really will be a memory_limit reached error, contact your server host and ask them to increase this value on your server!

Hostgator 

Hostgator has a common issue with their servers, that the given memory_limit value isn't used and your website stops around 100M memory limitation. They weren't able to identify the real cause at our customers, but upgrading the PHP versions from 5.x to 7.x solved the problem.