Thanks for your interest in this case.
Good news: PROBLEM SOLVED by swapping the device.
It was an issue of my device after all with a bunch of other bugs and errors that do not occure anymore on the new unit after excessive testing. BT, XT and BD Plastic engines are running steady and consistent between the loops, as expected.
If you got similar trouble, insist on swapping. In my case support did not tell me for sure, if it is only a unique problem.
Here you can see the proof 
The top one is the new one with the settings of my example BT_supp2_loop.zip
The bottom one is the same on the old device, as posted already. Very different.