I'm having an issue with the vanilla sharepoint 2010 navigation. In the root site and some of the sub-sites, I can edit/add/delete navigation items for the left nav with no problem. But for some reason, some of the sub-sites will no longer accept changes to the left nav items. I can add a link, hit ok, see it correctly updated in the navigation item list, and hit OK, all with no error. However, the navigation is not updated, and upon going back to the edit navigation page the link I added is no longer in the list. I get this same behavior when specify no targeting group, or 'All site users'.
The problem does not exist for any newly created sub-sites. Any ideas?