Add user / add email adress -> 502 Bad Gateway

Good morning,
when i want add client, customer, user or domain, or mail system make white page with

502 Bad Gateway
nginx

Why ?
Please help

i am using:

i-MSCP 1.3.16
Build: 20170107
Codename: Horner
SSL
LetsEncrypt plugin

system Debian

Yes, apache running, all my webpages running.

https://admin.mydomain.com:8443/client/mail_add.php

502 Bad Gateway
nginx

"GET /client/mail_add.php HTTP/1.1" 200 2226 "https://mydomain.com:8443/client/mail_accounts.php" "Mozilla/5.0 (Windows NT 6.2; Win64; x64; rv:51.0) Gecko/20100101 Firefox/51.0" "-"

yes, after service impscp_panel restart its work