Can not upload new app version

I can not upload new app versions. I follow the usual steps but after submitting the Upload app release dialog has the loading indicator spinning but nothing happens. After a few minutes I am checking my app page and the new version is still not up.

Browsing other pages I get

Proxy Error

The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request

Reason: Error reading from remote server

How can I contact the admin responsible for this?