It could be caused by a third party plugin conflict so please try testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Another step you can try is to increase your PHP memory limit. To do this, please edit your wp-config.php file and insert these lines: