You are seeing the error message “response is not a valid JSON response”
This can mean many things, but I see it a lot when a block or plugin is broken.
If you have the Gutenberg plugin installed, this is an experimental version of Gutenberg and could potentially break other blocks or existing blocks. You can deactivate the plugin and Gutenberg will still work (as it is bundled with WordPress as standard). Try that, as it may be that simple.
Otherwise, next time you see the issue please inspect the source code of the page and see what error messages are written, you can find out how to do that here – https://wordpress.org/support/article/using-your-browser-to-diagnose-javascript-errors/#step-3-diagnosis if you report them back here someone can try and diagnosis the issue some more.
