Thanks for the great response.
I now have bought a second ticket on LH from MUC to DUS 3 hours later of the first flight, for just 58Euros.
So I'll have 2 or 5 hours at MUC.
The cheap tickets cannot be canceled, but under EU-law if you cancel you get back the taxes and fees. And on the 58Euro-ticket the taxes and fees are 50Euro.