I've played around a little bit more. The mentioned rc.local "quickfix"
only works for the initial problem. If you have two or more
gateway-nodes and the initially "chosen one" disappears - bmx6 switches
to the another one. From this moment the routing-problem is back. So
somehow the routing-information is not "updated" on a
inet4-tunnel-switch/gateway-switch. Maybe this observation helps to
track this (imho last) big showstopper down.
greetings,
micha