Hi @fabrice,
Thanks for your hint. Yes, I have skipped the group command. Last time it worked as I simply combined the two signed transactions (simple cat) and opt-in has been done before. As a result it works even I have omitted the group. Now I follow the flow (unsigned txs => combine => group => split => signed in two nodes => combined => rawsend) and it works fine now.
Thanks again.
kc