Dear Sabine(if that's your name):
Although the flowers of this amazing Angraecum are considered long
lived, they are not going to be as long lasting as those of a
Phalaenopsis or lady slipper. We also don't know how long those
flowers were open before the show. The fact that the night fragrance
ebbed away would indicate the flowers are ageing naturally as you
thought, and not browning from stress. As these plants (or at least
all the ones I saw for sale) are first bloom seedlings, they will be
larger and have greater substance in succeeding years so can be
expected to last longer.
A few hints:
Never "water sparingly". When you water, water heavily, then allow the
pot to dry to the proper level before watering again. There is a text
on this site that explains all that.
Don't bother to mist your plants (you can mist aerial roots though) it
is a waste of time, and can cause more problems if not done early
enough in the day.
Sincerely,
Dennis