Ok, it's certainly something afoot with a firewall or some other hard to debug problem(maybe on the Kiwi side, maybe in the path somewhere)
My standard MTU breaks(physical 1500, advertised MSS 1460).
I'm able to receive full length packets but trying to send them to the web server means they never arrive as you can see by the retransmits until the remote times out and tells us to go away.
A reduced MTU on the route of 600(MSS 560) works.
1400 also works but I didn't capture it.
Edit: it almost feels like an old-school PTMU discovery failure like something in the path is blocking ICMP Fragmentation Failed because my outgoing packets are apparently setting "Don't Fragment"
Edit2: But disabling PMTU discovery and allowing fragmentation still fails.
Edit3: And my iOS device can post fine. Guess I need to figure out how to packet capture it.
Edit4: iOS is doing some deep voodoo it looks like, it sends the packet that gets retransmitted until failing on my desktop, but after 2 attempts it magically sends only part of the packet instead, which goes through fine. I'm not sure if Apple is doing some magic to deal with crappy links or what.
Edit5: Looks like the behavior is called "TCP MTU Probing" and when it sees packet loss it drops back the packet size. On Linux you can set "sysctl net.ipv4.tcp_mtu_probing=1" if this post goes through then that works.