Dear daltro,

this (default file browser in KDE's systemsettings) was in fact one of numerous things I tried at first, too. However, my instance of iceweasel does not care about this setting. (To make things even more weird, I have another PC at home installed with the same distro (debian) and version, having installed mostly identical package selections and with no wittingly differences in configuration, where everything works fine up to now.)

Hence, the needle in the haystack seems so difficult to locate without further ideas that for me the game of guessing might not be worth the candle. Of course, if someone had further clues, I'd be willing to test.