I have an old PowerBook G4 that worked (more or less) fine with a previous installation of Ubuntu Desktop 12.04. A few days ago I decided to install Ubuntu Server instead, and got a copy of Ubuntu Server 12.10. The installation seemed to complete successfully, but now, whenever I try to boot the system, it simply halts at some point after I unlock the hard disk. There is a lot of text on the screen (which is normal for me during a boot, except now it's mostly errors and debug information), the last of which is this:

[   26.338228] Fixing recursive fault but reboot is needed!

Pressing control command power to force a reboot yields exactly the same results.

A search for the error message turned up many temporary solutions involving kernel parameters, but none of them have worked for me. I don't think I can remove the default set of parameters (which I think is quiet splash), but I can pass additional parameters on boot. I've tried booting on AC and battery power, as well as using these combinations of kernel parameters while on battery power:

acpi=enable pci=noacpi pci=assign-busse acpi=ht
acpi=off
nomodeset
nomodeset acpi=off

Why am I getting this error and how can I fix it?