If you are having problems with apt and IPv6 a temporary solution is to use the option ‘Acquire::ForceIPv4=true’ by appending ‘-o Acquire::ForceIPv4=true’ at the end of the apt command.
apt-get update -o Acquire::ForceIPv4=true
If you want a more permanent solution you can disable the usage of IPv6 with apt altogether with:
echo 'Acquire::ForceIPv4 "true";' | tee /etc/apt/apt.conf.d/99force-ipv4
Sources:
https://unix.stackexchange.com/questions/9940/convince-apt-get-not-to-use-ipv6-method