The only differences to my code that I can spot directly is that I use MKT orders. Also I don't explicitly set the transmit flag (I think it is true by default). Further I already experimented with setting the tif to either DAY or GTC but it didn't make any difference.
After all my code can't be dead wrong since sometimes it works perfectly.
I will come back when I found the culprit.
However thanks for your comments! Really appreciated!
Despair