After setting up an IPv6 tunnel with Hurricane Electric I noticed a few things were not working properly in my Debian Jessie installation. The mtr network tool for example did not show any results and apt-get update stopped working (at least it made a long pause before continuing).
Thanks to a post from Ion there was an easy fix. Uncomment the line
precedence ::ffff:0:0/96 100
in /etc/gai.conf and the system will prefer IPv4 over IPv6. You don’t need to restart the system after the change, just save the file and try if that fixes your issue.