I’m now experiencing the same problem after updating. Ugh. I tried to isolate: I disabled EVERY plugin and switched to the default 2016 theme. I also updated WP again. Nothing has fixed it. Using Chrome.
The console errors I see:
https://www.derekbeck.com/wp-includes/js/tinymce/wp-tinymce.php?c=1&ver=4208-20151113 Failed to load resource: the server responded with a status of 404 (Not Found)
post-new.php:3081 Uncaught ReferenceError: tinymce is not defined
wp-langs-en.js:235 Uncaught ReferenceError: tinyMCE is not defined
post-new.php?post_type=page:3076 Uncaught ReferenceError: tinymce is not defined
Another forum said, try using the Google Libraries plugin. I did, this did not fix anything.
I’m also noticing a perhaps unrelated bug: clicking the checkbox to select all plugins to activate or deactivate them seems to work, but when you do the bulk activate from the drop-down, it goes through the motions but does not execute the command. But if you manually check multiple ones and use the drop-down menu to execute, it DOES work. So it’s as if the select all checkbox is only visually doing its job but not actually checking anything. Strange.