We experienced several times problems with hardware used for IPv6 which worked fine when used for IPv4 only! Some problems could be fixed when identified like the one where a network card was too slow to initialize multicast reception to pick up replies to router solicitation it sent on startup. However, others remained a mystery and we had to ‘fix’ them by switching to other hardware. Luckily the failures were with low-end Ethernet cards that were cheap to replace.