a test with: a clean 15.5 on a simple sata disk (ssd though) with mbr partitioning and all updates inside the normally working 15.5 and then zypper dup into 15.6 turns out the upgraded 15.6 working and booting normally. so I guess I am back to looking into why the upgrader or zypper dup messed up the kernel or initrd or whatever and booting parts? :(