The error “Updating failed. Error message: The response is not a valid JSON response.” is overwhelmingly seen in the new Block editor called Gutenberg.
The easiest way to resolve the updating error is by disabling Gutenberg and switching back to the Classic editor. As they say, old is Gold.
You can install the Classic Editor plugin for this. Once you have activated the plugin, try to save your posts. You should not see any error message now.