By default, Product Lists are set to sort by Sequence. But, as I have found and had to submit support tickets to solve, if some Product Categories have no value (null) in the Sequence field, this will cause the "Show Next 25 Products" button to fail. In these failure use cases, either nothing happens on button click, or a red box UI error message is displayed, reading: "There was a problem loading your category products. Please refresh the page to try to reload your category products. If the problem persists please contact support."
At the very least, this error message could more accurately reflect the problem (null values in Seqeunce field), but even better, there could be a validation rule on that field by default (since sort is by default sequence) that requires a Product Category to have a sequence value upon insert.