My problem was rather with problematic updates of zfs itself. The update "helpfully" updated initramfs for older kernels too... and if something broke, it brok

My problem was rather with problematic updates of zfs itself. The update "helpfully" updated initramfs for older kernels too... and if something broke, it broke previous versions too, so they all were unbootable. Eventually I ended up with an USB stick at hand with known bootable environment :(