The products list is fetched by the Product Recommendations engine and parsed. This parsing may fail, but there is no way to check this from the portal without contacting support (who also have no easy way of checking if there is an error).
This would save us and Opti support time if it was possible to check the status via some interface.
There is an interface in the Product Recommendations portal UI where you can see if the feed has failed. In the portal, go to "Admin > Site Management > Feed configuration > History". If you click on a failed feed, it pops up a snippet of the stack trace. In most cases, the preview of the stack trace should give a good idea of what the issue is, especially if it's a common one.
Forgot to mention that its usually the customer who notices this first when the recommendation widgets start showing too few recommendations, as products have expired or are out of stock.