Need help 404 errors on post URLs

Hi everyone, hoping to get some help with a problem I’m having on my WP site.

I logged in this morning to make a new post, and when I went Posts > Add New. I got this error:

Warning: Creating default object from empty value in
/homepages/4/d654779611/htdocs/newblog/wp-admin/includes/post.php on
line 708

All of my old posts content were still in WP, but every link I went to on the blog resulted in a page not found 404 error.

I backed up my blog files and re-installed WP, and theme. I also went back 1 day and restored the database.

Now when I try and publish a post I get this error:

Publishing failed. The response is not a valid JSON response.

The blog posts are also still not showing up on the site, even though all the content is still in the All Posts tab.

I should mention that this domains main website https://www.americanveteransaid.com uses a separate WP install and theme.

Been running these two WP sites on the same domain for about 3 years now, and never had a problem until now. Does anyone know what could be happening or how to fix it? I’ve tried everything.

Thanks for any help.

The page I need help with: [log in to see the link]