9-STABLE doesn't boot: can't load 'kernel'

[J David]

After installing 9.1-RELEASE amd64 on a system, it boots up fine.  If I
then build and install a new 9-STABLE kernel & world, reboots die in the
loader with:

can't load 'kernel'

This is a pretty straightforward system, one drive, not large (128GB SSD).
 GPT partitioned, gptboot boot code.  One UFS root partition to boot from,
a swap partition and, the rest for ZFS.

(At first I tried to do this system with root-on-ZFS but that also failed,
adding "unable to load zpool by guid" or similar before the "can't load
'kernel'" message.)

Once this happens, the disk is unbootable.  I can start from the install CD
and access the disk just fine, but even if I move kernel.old back to
kernel, it doesn't boot anymore.  Likewise, it doesn't matter if I
overwrite the boot code with gptboot & pmbr from the install CD or the new
ones from /boot after installworld.

The disk looks like:

# gpart show
=> 34 234441581 ada0 GPT (111G)
34 222 1 freebsd-boot (111k)
256 1792 - free - (896k)
2048 8388608 2 freebsd-ufs (4.0G)
8390656 8388608 3 freebsd-swap (4.0G)
16779264 217662351 4 freebsd-zfs (103G)

In the loader:
BTX loader 1.00  BTX version is 1.02
Consoles: internal video/keyboard
BIOS drive C: is disk0
BIOS 621kB/2067924kB available memory

FreeBSD/x86 bootstrap loader, Revision 1.1
(root at builder, Mon Apr 15 09:14:38 UTC 2013)

can't load 'kernel'

Type '?' for a list of commands, 'help' for more detailed help.
OK show
[…]
currdev=disk0p2:
[…]
loaddev=disk0p2:
[…]
OK lsdev
cd devices:
disk devices:
   disk0: BIOS drive C:
pxe devices:
OK ls
open '/' failed: no such file or directory
OK help
Verbose help not available, use '?' to list commands

So it's getting the boot device right (disk0p2 / ada0p2), but can't see it
at all.

Does anyone know what might be wrong?

Thanks for any advice!