Suggest you send the screenshot to Atlas for them to take up with their developer/host.
I am quite sure this is a server load issue as I often see it when I click on an email announcement right after it comes - but goes away in an hour.
We have a client with a 50,000 name Mailchimp list and when then send a mail promo out you get 5000 inbound connections in the next 4-5 minutes.
Huge, sudden floods of connections are. often mistaken for denial of service attacks and all sort of crazy errors not pointing at the root issue can get thrown...