Hm, the issue seems to be more complicated. I installed a new Tumbleweed QEMU VM in a SLES15 SP4 z/VM system. The 6.0.0-rc6 kernel boots fine there! For reference, this is qemu-6.2.0-150400.37.5.3. That means, I can currently reproduce the failure only in a fully emulated QEMU VM. Unless someone else can reproduce it in a native environment, I suspect this is a QEMU bug.