I brought this back to my web host and they investigated. Here was their reply so far:
Of the error logs that we looked through, not one was being written to while we visited and refreshed the site. Please keep in mind that the rest of the site also seems to resolve with no particular issue.
We also attempted to see if an error was being detected in the browsers "Developers Tools", but the developers tools also returned a "200 - OK" status, showing that requests were correctly being made to the server. To test this, we moved one of the "index.php" files to an unreadable filename, and installed our own "index.html" page with some sample text. This file resolved when being navigated to.
We believe that this might be the fault of the "AbanteCart's" configuration , but to confirm our suspicions, we will be passing this ticket up to our Level 2 Technicians to take a closer look, in order to ensure that nothing was missed over the course of our investigation.
Does that tell you anything by any chance?