Hello Igor,
thanks for your reply and the advice about the armbian-bsp* package. It worked! 👍
I tried it out and finally managed to update all previously withheld packages and get rid of the error messages, but it was still kind of bumpy...
I had to take these steps:
1. dpkg -r the armbian-bsp-cli-rpi4b-current because apt remove would not work and produce errors
2. apt --fix-broken install
3. dpkg -i --force-all /var/cache/apt/archives/armbian-bsp-cli-rpi4b-current_25.2.3_arm64.deb as apt install would not work without errors and dpkg would not work without forcing it. The error message was something about armbian-bsp* trying to overwrite a file that had been created by raspi-firmware (or something like that. Unfortunately, I did not copy the exact message)
Now, everything seems up to date, including the RPi firmware (which had been quite old in the pre-built image, now it's up to date). BTW: in my previous post I wrote that OMV is running on an RPi4B, in fact that was a typo, it is an RPI3B+ really, so the firmware is not in an EEPROM, but only in a couple files in the /boot/firmware folder.
Thanks again. I'm curious if it will stay stable or if future updates will cause trouble again at some point. 🙂 🙃