Hi Ludek,
I think you have created OnClick event in code behind file by double clicking on button or by some other way. Please remove that if this has been created. OnClick event it already in .xaml file. I would suggest you that please try to run application without changing it much.
I built this application using VS2012 and .net framework 4.5. I would suggest that please you should also use same IDE if you have. That will help you to reproduce this issue easily.
Please let me know if you still face this issue.
Regards,
Ravi