Heaps of kernel messages on the home server ndisc_send_redirect: not a neighbour which is ipv6 related. I did a google search for it which found a bit by Jonathan McDowell mentioning ndisc_send_redirect: not a neighbour but the real solution for me was in the comments. Somewhere eth0 of the server had lost its IPv6 address. Doing ifdown eth0; ifup eth0 made the correct ipv6 address reappear and now the messages seem to have stopped.