Hi Dominik,
I verified the checksum on windows with certutil, it seems to be ok
Theusb medium was formated and written thanks to balenahetcher
I'll try with another usb key
well, here I am on 8.1-2 and have the same issue (installation of package pve-firmware_3.9-1_all.deb failed). I hope I can find a less lazy answer somewhere.