Hi Alan!
Great job finding the help page on fixing 404 errors, it has a lot of good information that hopefully will lead us to the right solution. On that point, were you able to flush your rewrite rules? We find that fixes a whole lot of common difficulties. 🙂
As for functions.php, you will find that in your themes folder.
Since recurring events are working, and they have an extra element in the URL, it certainly seems related to rewrites. If neither of those paths helps you make progress, you might want to try switching your permalink settings to default.
Let us know how you make out!