Aha, that might be the problem. If you've owned a subscription in the past you were running the pro version. Now that you've downgraded, by installing the Core version (and it doesn't come with frontend submission) it is possible that the submission form is missing something from the PRO package.
Enable the error reporting and let me know what the error is. Maybe it doesn't have anything with the downgrade. But as I said - frontend submission is part of the PRO package and if you run the core version now it can stop functioning any time
You can find more information about the difference between the core and Pro version over here:
compojoom.com/joomla-extensions/hotspots#versions
Regards,
Daniel