Some comfort, yes, thank you. The tricky thing was that I coiuld not find any clue in the logfile: leaving out both empty curly braces generates no error message and the file typesets beautifully except for this one detail, a vertical instead of the desired horizontal arrangement of two figures. But when one leaves out only one of the empty curly braces the file does not typeset at all and there is a clear (well, fairly clear) error message. Like what happens for instance when one leaves out the {} for the caption after \placefigure[][].
A useful experience anyway. Thanks again.
Best regards,
Robert Blackstone