However, I finally discovered that the Wpbakery Page Builder is the reason for the 502 Bad Gateway error that haunts my site. I confirm this since if I deactivate this plugin everything is fine and no 502 error.
Sorry we have just got review 1* from you.
We helped you resolved many issues and now this is maybe server issues or you installed stranger plugin cause this.
Please resend us admin account and FTP account. We’ll check it for you!
We don’t think 1* is good when we help you resolved many issue.
You can create refund request here: https://themeforest.net/refund_requests/new