Finally got around to debug this:
Previously I had the system on an nvme drive and the bootloader on spi
After reinstalling the system and bootloader both on mmc flash, I eventually ran into the very same issue.
According to the logs, the boot bails with the following error:
Log1:
When I remove the nvme drive, the boot process works just fine for some reason:
(boot log after nvme removal)
Log2:
When I reinstall the nvme, the boot fails again. However, booting from sd-card (using recovery button) and reflashing the bootloader to the mmc fixes the issue as well with the same result as removing the nvme drive had, resulting in the same log2 as above.
I had fixed the issue before by reflashing the bootloader to spi but just like yesterday, it eventually stopped booting again until I reflashed the boot loader.
There isn't anything particular happening before this occurs, though the last few times it was always after the system had run for some time. After cutting power (without shutdown) and booting it again, the issue would appear. (The device is always cut from power instead of shutting it down, but there was never an error on any drive due to that).
The issue also did not show when the device was just turned of (without power) for a day or two. It seems to trigger quite randomly.
I already tried to erase the spi flash to avoid any old entries messing up the boot order but without effect.
Any clue what I could try next?