Thank you for your help, this non-sense was driving me crazy!
Unfortunately, this server is nothing special. It's an OVH VPS, with its default Debian install. Network is configured with OVH's DHCP. No proxy, but that does not mean OVH isn't mangling somewhere in the middle.
I'll reach OVH's bugtracker tonight, and update this ticket if there's something on their side.
Many thanks again, now I have a faint hope to make sense of all this. -- François Gannaz
