SharePoint was working for an existing site collection when accessed using the base URL e.g. https://oursites.internal.local/Communities/SP and was loading pages correctly. Something was broken recently such that the base URL was throwing a HTTP 404 - The webpage cannot be found - error even though all other pages in the site collection worked. There is no indication as to what caused this problem.
Solved by opening a page in the SitePages library then selecting the PAGE tab then selecting Make Homepage.
The site collection now opens correctly using the base URL https://oursites.internal.local/Communities/SP as expected.