The issue should be solved if you go to the nginx configurations and set the root directive to point to the <magento_folder>/pub. Don't forget to reload nginx afterwards.
After a month of debugging, we were able to find the cause of the problem. The issue was that backend process took 5 minutes to complete. For some reason the order confirmation email of Magneto took 4 minutes. Disabling all emails in the admin dashboard (Stores => Configurations => Sales => Sales emails) Solved the problem.
To debug the problem we ...
I solved this issue after server migration by query db & get custom admin url
Actually my env admin path not working for admin login url
'backend' => [ 'frontName' => 'myadmin' ],
http://www.magento.lan/myadmin 404 erorr
Step1: First Get Custom admin path
SELECT * FROM `core_config_data` where path like "%admin/url/custom_path%&...