Thanks, nelz, for your rapid and very helpful reply. I don't have any USB American keyboards, only PS/2, and was not really able to compare like with like.
Following your suggestion I looked into the BIOS and found:
USB Controller - Enabled
USB 2.0 Controller - Enabled
USB Keyboard Support - Disabled
USB Mouse Support - Disabled
USB Legacy Storage detect - Enabled
I switched the two disabled options to enabled and now everything works OK. Well, almost. In the BIOS the Y key (for Yes) is still in the US position, i.e. the Swiss Z key, but that is no problem.