Thank you all for the suggestions provided, and indeed it helps to disable IPv6 completely and remove the module from memory as follows:
Contents of the /etc/modprobe.d/ipv6.conf
alias net-pf-10 off alias ipv6 off options ipv6 disable_ipv6=1 blacklist ipv6
After a reboot, firing the command “lsmod” shall show no line entry denoting to “ipv6.”, and the same shall also be for the command “ifconfig”.
Optionally and if required, add in /boot/cmdtline.txt the parameter:
ipv6.disable=1
It works also for the latest April 2017 Raspbian image with kernel version 4.9.22-v7+.