I assume that the "print screen" key should launch spectacle. If I press it here (Plasma 5.5.2) it has no effect.
You'll need to delete ~/.config/khotkeysrc and re-login. This is a known problem.
Thanks for the hint. Sadly, it worked for me only partially. There seems to be a problem with some keyboard shortcuts on my system which stop working after some time after login. The Print Screen seems to be one of them (see https://bugs.kde.org/show_bug.cgi?id=357422, https://bugs.kde.org/show_bug.cgi?id=357419, https://bugs.kde.org/show_bug.cgi?id=356880)