PHP does not more anymore

Hello,

I update few plugins few days ago : yoats, modern tribe agenda, wpmudev . And till, our news site does not function anymore. In fact, the site work normally but we can not edit, publish, modify anything which require PHP. All php, java and post request generate a 404 error. I try everything : deactivate plugin, theme, install a new wordpress and, even, restore a backup few days before the issue, but the same problem goes on. Weird ! Could you please help me to install defender or have a look on that very annoying issue ? Thank's, Dr. Nicolas Roberti, Unidivers.fr