Unable to publish HTML Block after import from the library

Hello,

I’m using Gutenberg as the default editor. I tried several times to import templates from the library to the HTML Block (footer, to be precise) without success. Each time, I finished with the error message “Publishing failed. The response is not a valid JSON response.”

I even tested this in a brand-new dev environment; I got the same issue.

Any idea why this happens and how to solve the problem, please?