After a power loss, the Sheevaplug did not start up any more.
After some fiddling, I got it to work (ubifs needed to be mounted)
Then…
I attempted to upgrade U-Boot.
I wasn’t prepared to do a lot of migrating and attempted to downgrade and flashed U-Boot with garbage.

With Debian 6 I was able to run openocd and reflash U-Boot, as described here:

This will reflash the plug with a new U-Boot called uboot.bin (this is the .bin file and not ELF and the name must be uboot.bin).

> cd
> openocd -f /usr/local/lib/openocd/board/sheevaplug.cfg -c init -c sheevaplug_reflash_uboot -c exit

Wait 1.5 minutes.

It needs to be an older version of Debian, because the newest version rejects the JTAG part of the USB connection.