slaac via pppoe fails with sif6addr failed

trying to configure ppp0 via SLAAC by adding

+ipv6
ipv6 ,::3

and increasing the debug level, i find in the logs:

Nov 2 23:03:37 zotac01 pppd[3796]: cif6addr: ioctl(SIOCDIFADDR): No such address
Nov 2 23:03:37 zotac01 pppd[3796]: sif6addr: ioctl(SIOCADDRT): File exists (line 2607)
Nov 2 23:03:37 zotac01 pppd[3796]: sif6addr failed
Nov 2 23:03:37 zotac01 pppd[3796]: sent [IPV6CP TermReq id=0x2 "Interface configuration failed"]

ppp0 has two ipv6 ips, both local,
old ip remains linked
no default route is received
several IPV6CP ConfReq appear in the log not changing any parametr of the link
a further configuration of the interface via dhcp is not possible because the interface never becomes able to transmit ipv6 packages.

reads like https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=432728

i'm using pppd 2.4.5-5.1ubuntu2.2
source: https://launchpad.net/ubuntu/+source/ppp/2.4.5-5.1ubuntu2.2

lsb_release -rd:
Description: Ubuntu 14.04.3 LTS
Release: 14.04

cheers,

/markus