Can't add pages to menu

I recently updated to 3.9 and now I’m having trouble adding pages and saving pages to my menu. When I add a new page, one of the older pages disappears! When I try and re-save a previously saved page, it won’t save it. I also need to deactivate all the plugins in order to create and edit a new page. Deactivating the plugins doesn’t have any impact on the menu problem, however. Has anyone else experienced this problem? Any ideas?