Hi,
my project is quite big (174 files as of now) so I can't really put up the code here, I'll try to provide the essential parts (see below).
Nonetheless I would ask you to help me to understand a runtime error message:

can't find linker symbol for virtual table for `QString::Data' value
found `QString::shared_null' instead
This appears when I instantiate a QObject derived class. There are several QStrings being passed to the object with the construction.

The above error message appears when both of the following facts are given:
  1. One of the QStrings is empty (doesn't matter if I pass an empty String variable, "" or QString("))
  2. I have a breakpoint inside the object

A signal that should be emitted from that QObject is *not* being emitted in case of 1..

I have tried to narrow down the problem, but as of now it seems to be in the very moment of construction.

I thought it was perfectly normal to pass empty Strings - isn't it?

I have cleaned, run qmake, built everything new, restarted QtCreator, erased the -pro-user file - all to no avail.

Any ideas how to hunt down this problem?
I have a tendency of not knowing basic things because I've only started c++/Qt about a year ago.

Header of the abstract class:
Qt Code: 
  1. class AbstractApp: public QObject
  2. {
  3.     Q_OBJECT
  4. public:
  5.     AbstractApp(QString passedAppName,
  6.                 QString kursName,
  7.                 QString passedRoomId,
  8.                 MyModel* myModel,
  9.                 MyRoomModel* passedRoomModel,
  10.                 MySchedModel* passedSchedModel,
  11.                 ClassroomGraphicsScene *scene,
  12.                 QObject *parent);
  13. //and much more
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Implementation of the abstract class:
Qt Code: 
  1. AbstractApp::AbstractApp(QString passedAppName,
  2.                          QString kursName,
  3.                          QString passedRoomId,
  4.                          MyModel* parentModel,
  5.                          MyRoomModel* passedRoomModel,
  6.                          MySchedModel* passedSchedModel,
  7.                          ClassroomGraphicsScene *scene,
  8.                          QObject *parent)
  9.     :QObject(parent)
  10. {
  11. //much
  12. }
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Header of a derived class:
Qt Code: 
  1. class App_Raumbau : public AbstractApp
  2. {
  3.     Q_OBJECT
  4. public:
  5.     App_Raumbau(QString passedAppName, 
  6.                 QString kursName, 
  7.                 QString passedRoomId, 
  8.                 QVector<TischKlasse*> passedTischVektor, 
  9.                 MyModel* parentModel, 
  10.                 MyRoomModel* passedRoomModel, 
  11.                 MySchedModel* passedSchedModel, 
  12.                 ClassroomGraphicsScene *scene, 
  13.                 QObject *parent);
  14. //and much more
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Implementation of a derived class:
Qt Code: 
  1. App_Raumbau::App_Raumbau(QString passedAppName, 
  2.                          QString kursName,  
  3.                          QString passedRoomId, 
  4.                          QVector<TischKlasse*> passedTischVektor, 
  5.                          MyModel *parentModel, 
  6.                          MyRoomModel *passedRoomModel, 
  7.                          MySchedModel* passedSchedModel, 
  8.                          ClassroomGraphicsScene *scene, 
  9.                          QObject *parent)
  10.     :AbstractApp(passedAppName, 
  11.                  kursName, 
  12.                  passedRoomId,
  13.                  parentModel, 
  14.                  passedRoomModel, 
  15.                  passedSchedModel, 
  16.                  scene, 
  17.                  parent)
  18.  
  19. { //much
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I can assure the pointers are not the problem - the error message depends on empty strings:

Constructing in mainWindow.cpp (gives error message):
Qt Code: 
  1. app=new App_Raumbau("Raumbau", "Test", "", this->tischVektor, this->model,this->roomModel, this->schedModel, this->scene, 0);
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Constructing in mainWindow.cpp (gives NO error message):
Qt Code: 
  1. app=new App_Raumbau("Raumbau", "Test", "Test", this->tischVektor, this->model,this->roomModel, this->schedModel, this->scene, 0);
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