Can't boot or chroot after failed update

I run Ubuntu Hardy Heron on this computer.
This afternoon update manager failed after updating halfway, and presented me an error with the installation of the new libc for kernel 2.6.24-12. I tried running it again, because all the updates were still visible in the update manager, but it only reloaded the list when the Install Updates button was pressed. A check didn't hide the (possibly already installed) updates.

So I rebooted... and it stopped while booting.
Stopped right before init called login-related item (gdm with normal boot, tty login on safe-mode boot with "root" chosen from the list of options)

Choosing "resume" in safe-mode did bring up a login, but trying to login results in the following errors:

malloc: unknown:0: assertion botched
free: called with unallocated block argument

and some more errors. After this, it tells me the tty stopped working, and will therefore be restarted, followed by a new login.

Booting an older kernel (I still have 2.6.24-11, and several lower versions such as 2.6.24-3 installed) results in the same lockup (which can be resolved with Ctrl-Alt-Del though) or login error.

Booting a different rootfs (my older Gutsy installation) with the same /home (installed on a seperate partition on this machine) did work, but a chroot to the Hardy rootfs shows this error:

luitvd@LuitvD-laptop:/media$ sudo chroot ./sda2

malloc: unknown:0: assertion botched
free: called with unallocated block argument
last command: (null)
Aborting...Aborted (core dumped)
luitvd@LuitvD-laptop:/media$

NOTE: this was caused by the regular update-manager, but could be a general aptitude problem or a libc package problem, so I'm not sure which package to file this bug under...