Proxy Error - Media Temple

I've deployed a rails application to media temple's grid server (for the
first time) and seem to get this error (see below) after it's been up
and running for a day or so. It appears as though it happens without a
user action.

Any ideas?

-------------ERROR---------
Proxy Error

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

Reason: Error reading from remote server

Apache/2.0.54 Server at XXXX Port 80