what happens in your backend after saving the post, is the value for “weight” visible/saved or not? The first thing to try is always to check the plugins. Please deactivate ALL plugins and check if you can save the page settings. I remember a similar case with another client, the problem there was the hosting package with low “features” (less working space, that’s why the server did not handle all settings). But try first to deactivate the plugins, most of all problems are caused by plugins.