I installed FP Open 16.06 -> phone did not boot and started to heat up (recovery and fastbot was working, though)
So, I installed 1.3.6 -> still, it did not boot
Factory reset -> now it was booting (factory reset probably would have helped to boot the open os as well ... however ...)
So, I am now with the 1.3.6 OS and Irina's .DI.4.0.1-00161-M8974AAAAANAZM-1 Baseband firmware (at least this is what "About Phone" is showing). I have no SIM card and no SDcard installed. Still the phone is rebooting. -> Fuck.
Oh, by the way. regarding "Contact support": My ticket is open since sunday, no reply yet. And as I have to work during the day, finding a timeslot to call support directly is not easy, especially if I need my PC (fastboot, adb, etc) for the procedure.
I do not see, why it is so important not to share the method for downgrading. It is the only thing that seems to have helped at least one user.