Cannot embed content: ” The response is not a valid JSON response.”

Hello, and sorry for the bother.

I’ve been using v5.4.2 for a couple of days, and the embedding feature stopped working, in the middle of a post edit. Now, if I try to use the “Embed” block to add an image, YT video, Twitter post, or any type of supported content, I get a “Sorry, this content could not be embedded”, and then “Updating failed. The response is not a valid JSON response.” when trying to preview or save the post. When inspecting the element, I get a nice error: “api-fetch.js:838 GET 503”.

The content that is already embedded is still working fine.

I tried to:
-empty caches
-check for eventual mixed content
-use the classic editor (embedding images works but the rest still doesn’t)
-reset permalinks
-disable all plugins and use the twenty-twenty theme
-test older versions of WP in my sandbox sub-domain (the embedding feature stops working from 5.0 onwards)
-use Chrome, Firefox, and Edge
-various solutions (which I can’t remember) found on forums but with no results

I also tried everything listed in the “READ THIS FIRST WordPress 5.4 Master List” post, including the manual installation before writing this

I still can’t figure out what happened, since I wasn’t installing or updating anything when it stopped working (I refreshed the page, and BAM). I must have accidentally disabled or enabled something without knowing, but I haven’t been able to determine what.

Sorry about:
-the mediocre quality of my English
-asking what probably is a ridiculous question

Have a nice day/night

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