Miguu

I came across this post: https://passionwp.com/response-is-not-a-valid-json-response-error/
And this one: https://www.webnots.com/fix-slow-gutenberg-post-editor-in-wordpress/

Changed from Chrome to Firefox and the problem was solved.
Has to do something with extensions in Chrome in my case.
But the links offer multiple solutions. Hope it helps.

Just noticed it was an API issue! Solved.