Comment # 33 on bug 1160035 from Neil Brown

Sorry for the delay - I suddenly got busy :-(

The log shows the server getting error -95: EOPNOTSUPP
This is coming from kernel_sendpage()

I don't know what would be causing that.
I can think of two things to try.

1/ What does
    ip6table -L
  report.  I know you said you disabled the firewell, this will help me confirm
that it really is disabled.

2/ What does
    ethtool -k INTERFACE
  report for your network interface.  Give the interface name, not INTERFACE of
course.  For me, that is "ethtool -k enp4s0"
 I'm particularly interested in any transmit-offload features that might be on.
It would be worth using "ethtool -K INTERFACE FEATURE off" to turn each one
off, then see if that makes a difference.

If that doesn't get you anywhere, I'll have to ask a networking specialist for
help.