WordPress doesn’t save or publish new posts

This problem started when I wanted to create a new post. I’m using the Elementor editor, so I created a new post, set the title and featured image and then clicked on “Edit with Elementor” as usual. While Elementor was loading, I got this error.

My first thought was that this is an Elementor issue. But then I tried to first create a new post and then save it as a draft or publish it first before editing it with Elementor, and to my surprise, nothing happened. No matter what I do, I always end up with an “auto-draft” with no content which I can’t change for the life of me. Can’t change the name, slug, or to be precise, I can’t save any changes.

The funny thing is that this happens only for posts but not for pages. I can create new pages and edit old pages and posts without problems.

Tried deactivating plugins I installed since publishing my last post, clearing the cache, using a different browser, updating everything, but nothing worked. I’m not currently using a caching plugin and I see no errors in devtools.

Any idea what could be wrong?
I saw this question where they were doing something with the REST API, but I have no idea what that is, so I can’t tell if I can help myself with it.