There are some cases where the stack trace will contain PHI. To prevent this from ever happening QIE will no longer send stack traces in the email by default. If you want to re-enable this feature, you can go to the 'System Config' page, scroll down to the bottom and click on the 'Advanced Settings' button. From here you will scroll down to the 'Other System Settings' fieldset and then check the 'Email Stacktrace' checkbox. You will now get your stack traces in the error emails again.
Remember, enabling this option could potentially send PHI in the email alerts.