Publishing error. The response is not a valid JSON response. How to fix?

Question

WordPress

I started learning Wordpress, I know almost nothing. Tried adding an entry - when trying to post an entry, it throws an error: "Publishing failed. Response is not a valid JSON response."
Installed a plugin that checks the health of the site. It said: "The REST API was unable to correctly process the context request parameter."
The search does not produce anything useful for these errors.

Answer 1

look at the error log, it should say what gutenberg conveys there

Answer 2

Having gained experience, I saw an error on working sites. Sometimes solved by disabling Gutenberg in favor of the classic editor