Turns out this is almost surely due to a too limited firewall that
filters out PTMU ICMP packets in the Canonical VPN/network. Thus the
"please lower your MTU" messages never make it into the scalingstack
instance and into the container, and packets > 1400 bytes just end in
the void. I'll write an RT about this and add

  iptables -t mangle -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS
--clamp-mss-to-pmtu

to the instance setup scripts as a workaround until then.

Thanks Stéphane for explaining this!

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https://bugs.launchpad.net/bugs/1572026

Title:
  be more clever about MTU of lxdbr0

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