Please follow the following rules:
Reproduce issue on personal demo
Create personal demo https://www.rukovoditel.net/demo.php and reproduce issue on demo. Send link to the demo for me where issue is. Attach screenshots with the error and provide description how to reproduce this issue.
PHP error log
If HTTP ERROR 500 or blank page, find the PHP error log on your server and attach it. Usually this is the error_log file in the root folder, but other options are possible. Ask your hosting support where the PHP error log file is located.
JavaScript console
Probably a javascript error. Open the browser console (F12), repeat the action that caused the error and attach a screenshot of the JavaScript console.