Before wiping the entire XenForo file directory, and deleting the database:
And after re-installing, with(edit: without) any 3rd-party modifications:
Edit: I re-installed using a freshly downloaded copy of the zip (xenforo_1.1.3_full) from xenforo.com, eliminating the possibility of my changes affecting the re-install. I'll turn debugging back on to see if the error is the same.
Edit: The error is not the same. This time it's "A controller for the route path forum/index.php was not found."