Dear Fred
First thank you for your quick answer.
I now changed the IPv6 Prefix delegation to 0/64 (although my ISP speaks of a /56 subnet:
http://www.iway.ch/glasfaseranschluss-privatkunden/fiber-economy-fuer-privatkunden/luzern-ewl.aspx).
What I realized after taking one step back is that I didn't set any DHCPv6 DUID (which is bold in the form; but I haven't been provided with any).
Below you'll find all System Log events after booting (so no kernel messages) - maybe this helps getting the quirk?
Apr 15 17:44:21 rtadvd[279]: <ra_input> received RA from fe80::221:a0ff:fe56:e819 on non-advertising interface(vr1)
Apr 15 17:41:03 rtadvd[279]: <ra_input> received RA from fe80::221:a0ff:fe56:e819 on non-advertising interface(vr1)
Apr 15 17:40:48 dhclient: bound to a.b.c.97 -- renewal in 3343 seconds.
Apr 15 17:40:48 dhcp6c[296]: client6_script: child: exec failed: Permission denied
Apr 15 17:40:46 dhcp6c[271]: dhcp6_ctl_authinit: failed to open /usr/local/etc/dhcp6cctlkey: No such file or directory
Apr 15 17:40:46 dhcp6c[268]: client6_script: child: exec failed: Permission denied
Apr 15 17:40:46 dhcp6c[267]: client6_script: child: exec failed: Permission denied
Apr 15 17:40:43 dhclient: New Routers: a.b.c.1
Apr 15 17:40:43 dhclient: New Broadcast Address (vr1): a.b.c.127
Apr 15 17:40:43 dhclient: New Subnet Mask (vr1): 255.255.255.128
Apr 15 17:40:43 dhclient: New IP Address (vr1): a.b.c.97
Apr 15 17:40:43 dhclient: New Broadcast Address: a.b.c.127
Apr 15 17:40:43 dhclient: New Network Number: a.b.c.0
Apr 15 17:40:43 dhclient: DHCPACK from x.y.z.3
Apr 15 17:40:43 dhclient: DHCPREQUEST on vr1 to 255.255.255.255 port 67
Apr 15 17:40:43 dhclient: DHCPOFFER from x.y.z.3
Apr 15 17:40:42 dhclient: DHCPDISCOVER on vr1 to 255.255.255.255 port 67 interval 8
Apr 15 17:40:38 dhcp6c[189]: client6_script: child: exec failed: Permission denied
Apr 15 17:40:36 dhcp6c[123]: client6_send: transmit failed: Can't assign requested address
Thank you very much for your assistance!