Unable to boot - random lockups + specified root path /sysroot does not seem to be an OS tree

After a reboot of my TW machine I am no longer able to start it up
properly. The boot process seems to hang at various places around udev
cold-plugging devices. When it does not hang I get some sort of
emergency shell and I am informed that initrd-switch-root has failed.
The complete message is

Failed to switch root: specified switch root path /sysroot does not
seem to be an OS tree. os-release file is missing

Some notes:

  • This is a Lenovo W520 Laptop
  • I have previously booted this kernel ( 4.0.3-1-desktop )
  • I am using the encryption setup suggested by Yast ( ext4 /boot
    partition IIRC, / is btrfs )
  • I get the prompt to enter my passphrase and it is accepted
  • booting without ‘quiet’ shows that the hang occurs sometimes after udev starts
  • I am using the nvidia binary driver
  • I sometimes get the hang after the screen dims a little and the last
    entry printed is ‘ACPI: Sleep Button [SLPB]’ . Booting with acpi=off
    from the ‘failsafe’ boot entry does not make the hang go away
  • I think I am able to get into the emergency shell about 2/10 of the times
  • I tried booting with nomodeset and
    modprobe.blacklist=nvidia,nvidia-uvm, with no luck
  • I have switched to using the integrated graphics card ( Intel ) from
    the BIOS menu, but that did not help

I’m really out of ideas about why my machine does not work anymore, so
any help is more than welcome.

By using a TW live CD and booting once my encrypted partitions were unlocked I realized that my btrfs partition had the wrong default subvolume id set and therefore mounting it resulted in getting an invalid root filesystem ( it was actually a wine installations ).

Once the default subvolume was set up all was fine.