As a temporary work-around on my ubuntu machine, I added the following line to /lib/systemd/system/NetworkManager.service in the [Service] section:
ExecStartPre=-/bin/sh -c 'rm -f /var/lib/NetworkManager/dhclient6-*.lease'
Now I get DHCPv6 leases again, even if my machine was powered off for some time. This probably doesn't work for suspend/resume, but is better than nothing.
As a temporary work-around on my ubuntu machine, I added the following line to /lib/systemd/ system/ NetworkManager. service in the [Service] section:
ExecStartPre= -/bin/sh -c 'rm -f /var/lib/ NetworkManager/ dhclient6- *.lease'
Now I get DHCPv6 leases again, even if my machine was powered off for some time.
This probably doesn't work for suspend/resume, but is better than nothing.