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I installed Zen Cart on XOOPS 2.0.13.2 and it seemed to work, but I confess I didn't test downloads.
There is an .htaccess file in the 'download' folder that is designed to recognise a logged-in user. You could disable this temporarily to see if that is the problem.
In line with the trend
the developer has set up an independent support site at:
http://dev.imaginacolombia.com/but sadly most of the forum posts seem unanswered.
Ecommerce modules seem a nice idea, but to protect against fraud you need full customer details on record, unlike the basic and essentially anonymous user profiles used by XOOPS and other CMS. Unless the module creates an integrated registration system, people have to register twice. I can't see the advantage of using a module compared to using a stand-alone ecommerce installation.
Personally, I would install the latest Zen Cart, hack the template to look similar to your XOOPS site and put a link to 'shop' in the XOOPS menu. Upgrades to XOOPS and Zen cart can then be undertaken without worrying about any current or future incompatibilities.
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