We had a similar issue; editors could not switch between visual and text tabs, and the page content often didn’t even show at all.
In our case it was a bunch of Javascript errors. When Javascript fails, lots of things on the editor page don’t work. Look at the browser console (available via F12 dev tools in most browsers) for Javascript errors. Each error should show what .js file it occurred in (try hovering over the name if you don’t see the full path.)
Look for files inside wp-content/plugins, and try temporarily deactivating the associated plugin to see if it helps. We just needed to deactivate Yoast SEO, but it may be different for you.
