Finally found the mistake
Hi Eric,
found the mistake, and as it seems it has been on my side: I forgot to select the key column in the Recordset, but it needs to be selected - of course!
It is not enough to select it in Data Assist's multiple update and/or multiple delete server behaviour, you also needs to select it in the adjacent recordset! Thats where my misunderstanding happened.
Thank you for your patience!