@John VV: no-script is not installed.
Update: today all buttons work! Why? I have no idea. I had rebooted before, and it did not help. Apparently the computer needed a good, relaxing sleep
I would still like to know what had happened (in case the issue reappears), but for now everything is ok. I do not plan to write 'solved' in the title, as this thread does not really provide a solution.