I also was getting this error:
Code:
An unknown response null: :text/html; charset=utf-8: :SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data was received while processing an ajax call. The action you requested could not be completed.
I found this post:

Originally Posted by
stecrox
Anne
I found the answer and the issue was also evident on the Sheffield Responsive Template. Unfortunately the answer did not come up when I searched for it. After going through a few pages I found what appeared to the be answer and yes it worked.
I removed the following file: includes/templates/winchester_responsive/jscript/jscript_framework.php
Thought you would like to know.
This solution also works properly for the Winchester_black. Thank you for posting this solution!
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