Thanks for the quick reply! Just to be sure: I definitely boot into Fastboot mode and that’s when I test the fastboot shell command. As I said, I know how to identify it. I was just asking about TWRP to be sure that the version of TWRP I have flashed on my phone does not somehow interfere with fastboot. And I take from your reply that it doesn’t matter which TWRP I have installed since they are two fundamentally different modes and don’t “talk” to one another.
Re cable: I have performed file transfer with it numerous times and also very recently.
Thanks so much for the last point re fp2-gms-19.11.2-manual.zip that it is not suited to be installed with TWRP. I had followed this guide but it seems that a zip file must have a certain structure as I found mentioned here. In the mentioned guide I am referred to use the zip file from here. I then also tried the versions OS 18.04.1 and OS 18.09.2 found here. I guess needless to say that I get the exact same issue.
Do you know where I can find a zip file suited for installation via TWRP? I am skimming the net but not sure what to search for.