Comment 4 for bug 153991

There seem to be several people in the ubuntu forums with that problem. I have a similar one (probably related):

I've build a custom 2.6.22 (standard gutsy kernel). First without initrd support, which worked with 2.6.20. But it didn't compile. So I enabled initrd support and removed the initramdisk line from the grub config, otherwise I have the above mentioned problem (busybox...).
Don't know if this helps.