boot partition not found

I have installed 20210401-0112-postmarketOS-v21.03-i3wm-0.3-nokia-n900.img to a 32GB SD-Card plugged in to my N900 and booted from SD-Card. After some kernel messages, tons of messages like this "mmc1: unspecified timeout for CMD6 - use generic" appear - and finally the message ""boot partition not found""

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