Using openSUSE Leap 42.2, but IIRC also with SLE 12 SP2 on other hardware, I am
unable to boot. The root filesystem can't be mounted in the initrd, the message
is "/dev/sda2 or /sysroot is busy" (written out of my head).

A manual mount is indeed failing as well. I can boot when I remove the
root=UUID=....   parameter in grub. However, the used UUID there is the same as
in /dev/disk/by-uuid/

The setup is simple, just two paritions on my SSD in my notebook.

Not sure where to look next, any hint?