Add Post page barely works

After I upgraded to the latest version of WP nearly all of the functionality on my Add Post page stopped working. I can type in a title and content and post it, but that’s it. The formatting buttons don’t work, when I try to create a hyperlink it does nothing, I can not schedule the post for another time, I can’t add a tag, etc. etc.

I already updated my theme, tried disabling all of the relevant plugins, but still no luck. Any suggestions would be greatly appreciated. Thanks.