Cannot update menu items

My site is working fine and allowing edits in all respects, except that one of my two menus --- my home menu that lists basic navigation choices --- will not permit me to add a new link. Actually, I want to delete one item and replace it with a new one. I have deleted the old link and "Published," but Euphony will not allow me to add the new link, either from the existing list of unused choices or by creating a new custom link. When I try to "Publish," I continually get an error message, "Looks like something’s gone wrong. Wait a couple seconds, and then try again." I have disabled all plugins, which several sites recommends, but that has no effect. I have also upgraded to the latest version of Euphony Pro and have updated all plugins. Again, the result is the same. Chrome's console, if I'm reading it correctly, says "Failed to load resource: the server responded with a status of 406 (Not Acceptable)" and it seems to be connected to admin-ajax.php. (I don't have the expertise to understand what that might mean.) Is this something that is easily fixed? Thanks in advance.