There’s a post from user “ropebih” showing his dificulties on ccr with ipv6 where dhcp server gone( http://forum.mikrotik.com/t/6-x-on-ccr-invalid/75473/1 ), the problem happens here too(dell server). It occurs when whe use DHCPv6-PD pool option in pppoe profile. The pppoe conects them send the ipv6 pool and if the pppoe conection drop the dhcp-server gone and only solutiuon is to reboot the router.