new year's greetings,
As I don't need ipv6 for the moment, I removed the following packages and my 2 C7V2 seem to be stable:
opkg remove ip6tables
opkg remove kmod-ip6tables
opkg remove kmod-nf-ipt6
opkg remove kmod-nf-reject6
opkg remove odhcp6c
opkg remove odhcpd-ipv6only
Maybe it is just some magic coincidences.