After setting the debug flag '-d' and redirection stderr to stdout (2>&1) I get the following message in /var/log/messages: popularity-contest: unable to submit report to http://popcon.debian.org/cgi-bin/popcon.cgi. Also, the cron job output 'popcon Timed out.'. So I guess the HTTP error is a server timeout. I see the timeout in 'popcon-upload' is 30 seconds. Maybe increasing that to 60, 120 or 300 might help?