I have been running Zen Cart successfully for over a year now. However, all of a sudden (without me making any modifications to anything) an error starts popping up at the top of every single page. The error is:

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Not Found
The requested URL /index.php?main_page=product_info&cPath=1&products_id=3 was not found on this server

Apache/2.0.61 (Unix) PHP/4.4.7 mod_ssl/2.0.61 OpenSSL/0.9.7e mod_fastcgi/2.4.2 DAV/2 SVN/1.4.2 Server at store.amainyc.com Port 443
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At the same time, I also noticed that on product pages, the "New products for the month of April" started popping up even with products that are no longer available.

It's not a template issue because I've switched back to the default "Contemporary Green" one and still see the error at the top of every page.

Can someone please help diagnose this one?
My cart is located here: https://store.amainyc.com/

Thank you,
Andrew