Jens:
Run the stty -a command and look for what character sequence sends the quit signal (signal 3).
On one of our linux systems it is:
quit = ^\
^\ is character 28 (decimal)
From what you have described you should see:
quit = ^^
^^ is character 30 (decimal)
To recreate your issue, I :
stty quit (this may vary depending upon your terminal emulator...
