@chad Yes, it ist ;)
@asherkin Thanks, yes that is the doc I was linked to, and indeed, putting the following in the preamble solved it:

$_SERVER['HTTPS'] = true;

Sorry, I said it was late ;)
However, I don't consider this a special installation (and other apps on the server don't show such behaviour), so I find it kind of "interesting" that I have to resort to such measures?
Thanks