I couldn't find any information about this error anywhere and it's really strange!
When I open the root-level Navigation settings page in my SharePoint 2010 website, I get an error page and the following message can be found in the Event Viewer: Exception type: InvalidCastException Exception message: Unable to cast object of type 'Microsoft.SharePoint.Publishing.CachedListItem' to type 'Microsoft.SharePoint.Publishing.CachedPage'.
I've tried clearing all of the SharePoint 2010 related caches but that didn't work. I also haven't modified any part of the root level navigation in a long time. I think the last time I opened the page was a month ago or so!
The master page I'm using is is v4.master.