I found the issue and I thought I would report it for future users of the /quitcommand parameter in the <defaults> element. In my main script that includes the survey I specified /quitcommand = (56 + 30). In contrast to what the manual suggests, the words Ctrl and Alt don't work at all ('not valid') so I used the scancode for 'Alt' which is 56.
I figured this was the issue since the crash happened only when I typed the letter 'A'. It seems the quit command was interpreted as the 'A' key only. I have removed the quitcommand completely and it runs perfectly- no crashes so far. However, I need to specify a different quit command so that when the participants want to quit they go to the finish page, instead of simply quiting the program. What are combinations that work in the /quitcommand, if the command keys are not considered valid anymore?