Can't create new posts

Hello! I’m having a big problem on my site and it just appeared out of nowhere. For some reason, I am completely unable to publish a new post. I can click “add post”, type in content, etc., but when I hit “save as draft” or “publish” it goes immediately back to the “posts” page and I lose all my content.

Additionally, the “new posts” page is wonky in general — categories are missing, tags are missing, some of the plugins (like Yoast SEO) are empty, and the comments section at the bottom says “an unidentified error has occurred.” If I try to create a “quick post” from the dashboard, the content of the “quick post” will erase and replace the content of an already published post rather than creating a new post.

I’ve tried to create a new post on both sites running under my multisite and they’re both having the same problem.

I CAN edit other people’s posts. I just can’t create new ones.

I did absolutely nothing to bring this on — no updates, no adding plugins, nothing. This came completely by surprise. We have tried it with various users — admin, editors, contributors — and everyone is experiencing the same problem. It’s quite unsettling, especially since my core business is my website and it’s been down for nearly a week!

Things we’ve tried so far:
-Disabling all plugins
-Deleting all recent comments wondering if there might be faulty code
-Switching the theme back to 2014. Switching the theme to 2014 and disabling all plugins at the same time.

If anyone has any insight I’d appreciate it!