an inside LAN machine establish a V4tunnel with a internet provider to have an IPV6 address, the connection establish, ipv6 address is valid.
after a while (long while, few days maybe) the IPV6 tunnel becomes dumb.
no more ping -6 to any ipv6 address. At this time it is impossible to bring up the tunnel again, except if I go to mikrotik router and kill the existing connection, the only one of type 41 in the connection list. after killing the connection, tunnel is able to bring up again.
... filling in a bug report.