Save Menu gives 404 error

Hoping someone can please help.

Had a site running just fine for more than 12 months. Out of the blue, it starts spewing 404 errors when trying to save a menu.

I’ve read just about every solution discussed in these forums. Modifying .htaccess rules, resetting the plugins folder, disabling plugins, rolling back to the default template, running with debug on to see if anything happens…nothing. Still the same persistent issue of not being able to save a menu without getting a 404.

I have several other sites on the same server, all using the same plugins and templates (Genesis + Child Theme and a couple of PluginBuddy things). None of these sites have the same issue.

Nothing new has been added to the site, other than content (pages).

If someone could point me in the right direction of the solution, I’ll give you my first born child!

Thanks