Unfortunately it’s only a workaround (but I will write more as soon as possible) but this post pointed me to just disable “Load system GDB pretty printers” in Tools->Options->Debugger->GDB
This behavior is a filed bug in QtCreator 3.0
seems to work for me now!
EDIT: Although the bug tracker for QtCreator 3.0 says the bug has been fixed, the bug actually persists in later versions – at least as of v3.1.2 . Fortunately the same suggested workaround applies.