Symptoms
Unrendered page (code in plain text) is displayed in Firefox when the link with the PromoCode is followed for the second time in the same browser.
In the Internet Explorer the browser asks to save a json/php page instead of opening the store.
Cause
The issue is caused by the bug with id #PBA-62292.
Resolution
Modify /usr/local/bm/templatestore/libs/3rdparties/JSON.inc by commenting the line:
header('Content-type: application/json');
And then restart apache on the store server.