"The program has unexpectedly finished." after successful execution

[gib]

When executed from within Qt Creator, my program does everything it is intended to do, but when I terminate it by clicking the X button, it hesitates for about 5 seconds then shows this message in the Application Output window:

The program has unexpectedly finished.
E:\spheroid-abm\spheroid_GUI-static-build-desktop\release\spheroid_GUI-static.exe exited with code -1073741819

In main.cpp I check the return code from app.exec() and it is 0.

res = app.exec();
return res;

If I run the executable outside of Qt Creator I do not get the unexpected finish message. It seems to be something about Qt Creator. I am running version 2.4.1, built with Qt 4.7.4 (32bit), on Windows 7, using MS Visual Studio 2005.

The above error-free execution does not apply on another Windows 7 machine. On this one the program runs correctly, but when it is terminated by the 'X' button, when executed either from within Qt Creator or from the .exe file, there is a pop-up:
spheroid_GUI-static.exe has stopped working
then options for check online, close or debug.
The value of res is still 0.