And the most surprising thing is, that it worked for one single moment, see the PS, and stopped working after the next reboot - with all what I tried to make it work still untouched and in place.
Any further tipps here?
do you even have an IPv6 service from your ISP? try pinging [2a00:1450:401b:801::2004] (an address I get for www.google.com)
also, ping now has the -4 and -6 options to specify which protocol to use. otherwise, AFAIK the resolver in glibc autodetects if it'll use ipv4 or ipv6 by defult