Comment 32 for bug 1254085

This solved the issue:
Append to /etc/sysctl.conf the following:
net.ipv4.tcp_mtu_probing = 1

after restart you should see at /proc/sys/net/ipv4/tcp_mtu_probing the value "1"

A temporary solution is:
echo 1 > /proc/sys/net/ipv4/tcp_mtu_probing
caveat: this will be reset at boot.

You can try also with value "2" if still not working.

(see explanation at:https://thesimplecomputer.info/pages/adventures-in-linux-tcp-tuning-page2)