Hey Ivan, I figured out what change was causing this. Our client was requesting extension-less URLs so after doing a bit of researchy, I changed:
<cms defaultProvider="Sitefinity" pageExtension=".aspx"
<cms defaultProvider="Sitefinity" pageExtension=""
and the problem disappeared.
Do you know how we can achieve this functionality?
In addition (thanks for your patience!), I want to implement lowercaseUrls and change the whitespace character. This doesn't seem to take effect until I've saved that particular item again. Is there a way around this?