Shift key stops working after a while

I'm using Qt Creator and after a while the shift key stops working, e.g. it's not possible to write capital letters anymore (...and other characters that can be triggered via Shift).

Observations:

1. For other Qt and Non-Qt applications it is still working
2. Also, restarting Qt Creator fixes the problem for a while
3. I've debugged this a bit a QLineEdit/QWidgtLineControl. A breakpoint at qwidgetlinecontrol.cpp:1875 (line with "insert(t)" in the function "processKeyEvent", t = event->text()) shows that:
   1. Working case: t is "A" if shift is pressed
   2. Non-Working case: t is "a" if shift is pressed

This already the case at QWindowSystemInterface::sendWindowSystemEventsImplementation() (qwindowsysteminterface.cpp:581, "QGuiApplicationPrivate::processWindowSystemEvent(event);") in the stack trace.

I can reproduce the issue reliably only in the debugger.

1. Start a Hello-Worl-GUI app with a QLineEdit (e.g. create a "Qt Gui Application" with Creator and drah a line edit on the form in the *.ui).
2. Set a breakpoint at qwidgetlinecontrol.cpp:1875 (function QWidgetLineControl::processKeyEvent, line contents: "insert(t);", t = event->text())
3. Run the application
4. Press Shift-a - breakpoint is triggered, inspect "t". It's "A".
5. Continue debugging
6. Press Shift-a again, breakpoint is triggered, inspect "t". It's "a".