user's arguments are empty with QCoreApplication in mysterious cases
Q(Core)Application wants argc and argv by reference, so your constructor should read
Diagramm(int& argc, char **argv[])If you don't to this, it may work in some cases and lead to segfaults or strange behavior in others, as you encountered. Seems to be a common error and isn't easy to spot when reading the documentation.
arguments() is a static function so the line should be:
QStringList arguments = QCoreApplication::arguments();