Strompi 3 - V1.72 - Serial Port always active after running Start_Serialless.py

Hello there,
Is there any reason why the Strompi 3 still answers on the serial port (minicom -s on /dev/serial0) after I enabled the "serialless" mode using sspc/quit , and run "Start_Serialless.py"? This prevents me from using the "ultimate GPS HAT" that uses the uart. Of course the GPS HAT is not placed during the following explanations:
I used GPIO_TPIN = 19 and soldered pin 20 on the reset pad, reset jumper still on (as described in the user manual),
and the "powershutdown_serialless.py" script, also using the GPIO_TPIN = 19, works.
I start "Start_Serialless.py &" followed by "powershutdown_serialless.py" in /etc/rc.local as recommended in the user manual. I also run "Start_Serialless.py" in the command line, but strompi is still active on serial port.
Strompi 3 freshly flashed with V1.72 with V1.72 scripts used.
Do you have hint what I could have done wrong?
Should I use a separate pin for "Start_Serialless.py" than "powershutdown_serialless.py"?
Otherwise it's a nice little PSU and I'm eager to have a look at the open code at some point!