As I said this just started happening.
Nothing could "just start happening" specially in IT world. Everything has its reason here...
Could that possibly be what's wrong that I mucked something up in the cookie file?
Surely it could be possible, if you modify scripts without obvious understanding what will happen.
The notice you mention, is not related to $allowHyperlinks option at all. It comes from the Anti-Guest add-on
which you obviously have installed, if you're seeing this. Try to modify options of this add-on or completely uninstall it.