From the original question submitter:

Investigation (thanks for the help, atcroft!) reveals that if I force the action of the button to be "index.cgi" (second argument to the start_form routine), it works okay again. This should be the default action, so I don't understand why it broke in the first place.

I have a workaround to make my customer happy, but I'd rather not see that index.cgi at the end of the URL, so I would still like to solve the root problem if at all possible.

Advise is still appreciated,
Kurt Leucht