The laptops had build 656 installed, so I updated them, using the Terminal activity:
$ su -
# olpc-update 767
So far, so good. After rebooting, the laptops insisted on updating their flash memory. This requires a connection to the power supply.
Which I did not yet have (the laptops came with US plugs). I have ordered simple adapters for use in Europe, but had not received them yet. So, after rummaging in my box of power adapters, I found one that fit the laptop, and everything went fine.
So be sure to have a power supply before doing a software update...