After a hard job, finally I find the reason and solution for above error and here I want to share it with others.
The reason for above error was blocking the crawl IP by firewall in the Cloudflare and it is not related to username and password in our feed configuration. If you face same error, first check the firewall setting in server and your Cloudfare. If the IPs from which the Google Merchant is trying to connect has been blocked, you face above error while fetching your feed and you need to add this IP to your whitelist or unblock it. Also you can check your geographically block list too, if you block USA, you may face this problem, because Google Merchants IPs change periodically and you will face this error before indexing and recognizing the new IPs by Cloudflare.
My problem was blocking USA in Cloudfalre and Google Merchants IPs changed last week, and the new IP was not indexing by Cloudflare as a Google crawls so it cannot reach our server.