Raspberry Pi 4 image not working on Rev 1.5

[sleeper-ds]

As already described on Discord on 2022-06-25, according to dmesg, I have a Raspberry Pi 4 Model B Rev 1.5. It is the 2GB version. When trying to boot the current (v1.91) image, the green LED just blinks 4x long/4x short over and over, which seems to indicate an unsupported board type (found after some googling).
I had the Pi connected via a USB/power splitter as well as directly to an USB-port (hoping that it could provide enough power), both times with the same result. The Pi and the card seem to be ok, as I could install and run Raspberry Pi OS with no problems (and get the board revision from dmesg).