@rmax and all others having trouble with unstable fastboot mode:
I have tested the fastboot mode of my FP4 with mAid_v4.2-full_x86_64.iso running on my PC. And although the Manjaro linux that is installed on my machine and also the fastboot version of it is newer than the software from that iso (which is also based on Manjaro), the fastboot mode is stable when i use this mAid system on my PC and remains unstable with my own newer Manjaro !
So the instability for me definitely came from the software on my PC and not from cabling or specific USB and i advice to use current/actual mAid live OS for fastboot or adb sessions with android devices.