Hi Jeff, I did some modifications because the answer to the spam challenge question was showing up in the input box, set as the placeholder. Also, some backslashes were strangely being added to my question so I added the question directly in submission-form.php.
I just tried uninstalling and reinstalling. It now works for me with default settings. The spam challenge answer is still showing in the input box as the placeholder. To fix I removed this on line 65 of submission-form.php:
placeholder="<?php echo $usp_options['usp_response']; ?>"
It's now working perfectly, so maybe I messed something up when I was previously modifying submission-form.php. Thanks for this useful plugin!