[bitfolk] "Boot loader didn't return any data!" starting Xen VM

Before I start, I should say that this problem doesn't actually relate to my Bitfolk VM, but I hope it's OK to mention it here. It does relate to Xen and I suspect there are a lot of people here who use Xen. I have a non-serious Xen host at school, used to let pupils do recreational programming. One Xen host (running on a Mac mini) with one guest. Both host and guest run Debian Squeeze, and the configuration has been stable for a long time. This morning I applied the latest security updates and re-booted them as follows: * Upgrade guest * Shut down guest * Upgrade host * Re-boot host The re-boot was because the updates included a new kernel. I expected the guest to come back up as normal but it didn't. Trying to start it manually I got: hydra:/etc/xen# xm create pragma.cfg Using config file "./pragma.cfg". Error: Boot loader didn't return any data! hydra:/etc/xen# Doing a web search for this error message produces lots of hits, but very little information. xend.log contains lots of debug information from xend, but as far as I can see none from Pygrub. xend invokes pygrub, pygrub doesn't do its thing, xend then halts the process. Both systems are backed up nightly so I have very recent backups. I can mount the guest's root file system on the host and examining it nothing significant seems to have changed, and certainly all the obvious files which were there before are still there and still in the same place (although some are updated versions). It's unfortunate that I did both upgrades at the same time, because I now don't know which one is causing the problem. Can anyone give me any suggestions about how I could get Pygrub to give me more information on what it is unhappy with? TIA, John