[MM3-users] Apache proxy error messages.

I have two users who are getting a proxy error message when they try to connect her our mailman server: 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 Apache/2.4.52 (Ubuntu) Server at lists.psfc.mit.edu Has anybody else in an errorlike this before? One person manage to get around this by going in incognito mode on the browser. Does that make any sense?