open
/includes/classes/sniffer.php

Around line 50, you'll see:
Code:
   $debug1=false;
TEMPORARILY change that to
Code:
   $debug1=true;
Then open your Zen store in a browser.

Then quickly go back and change the setting to false again so that all that important info isn't shown to the whole world.

Then use that output to find out what stage it's failing at.

You can even post the output here ... just blank-out the passwords for your own protection.