In the meantime I overcome this issue.
I found a Github repo documenting a different command for starting QEMU:
qemu-system-arm -serial stdio -M versatilepb -cpu arm1176 -m 256 -drive "file=DietPi_RPi-ARMv6-Buster.img,if=none,index=0,media=disk,format=raw,id=disk0" -device "virtio-blk-pci,drive=disk0,disable-modern=on,disable-legacy=off" -net "user,hostfwd=tcp::5022-:22" -dtb /home/thomas/Hardware/Raspi/versatile-pb-buster.dtb -kernel kernel-qemu-5.4.51-buster -append "root=/dev/vda2 panic=1 rw" -no-reboot
Unfortunately the emulated Rpi has no network connection, but this is a different issue.