Found the culprit: apt-cacher-ng which I use as a local cache
From the log:
Sun Mar 2 09:39:23 2014|Returning to last state, 1
Sun Mar 2 09:40:26 2014|Detected incoming connection from the TCP socket
Sun Mar 2 09:40:26 2014|Client name: ::1
Sun Mar 2 09:40:26 2014|Raw request URI: http://keyserver.ubuntu.com:11371/pks/lookup?op=get&options=mr&search=0x15331454B72FD7EC
Sun Mar 2 09:40:26 2014|Prepared response header for user:
HTTP/1.1 403 Prohibited port or config error (double proxy)
Content-Length: 216
Content-Type: text/html
Date: Sun Mar 2 01:40:26 2014
Server: Debian Apt-Cacher NG/0.7.2
Connection: close
apt-cacher-ng is version 0.7.2-1ubuntu2.1 from precise-updates
Found the culprit: apt-cacher-ng which I use as a local cache
From the log: keyserver. ubuntu. com:11371/ pks/lookup? op=get& options= mr&search= 0x15331454B72FD 7EC
Sun Mar 2 09:39:23 2014|Returning to last state, 1
Sun Mar 2 09:40:26 2014|Detected incoming connection from the TCP socket
Sun Mar 2 09:40:26 2014|Client name: ::1
Sun Mar 2 09:40:26 2014|Raw request URI: http://
Sun Mar 2 09:40:26 2014|Prepared response header for user:
HTTP/1.1 403 Prohibited port or config error (double proxy)
Content-Length: 216
Content-Type: text/html
Date: Sun Mar 2 01:40:26 2014
Server: Debian Apt-Cacher NG/0.7.2
Connection: close
apt-cacher-ng is version 0.7.2-1ubuntu2.1 from precise-updates