After installing, updating or activating a plugin you may find you suddenly cannot save or edit WordPress posts or pages. You can see, open, edit blocks and text but when you finally click “Save” you are suddenly prompted with a vague, unhelpful banner toast: “Updating failed. Error message: The response is not a valid JSON response.”
If you encounter this situation, it can be relatively easy to fix but before heading on to that, FIRST copy/paste the edited post or page to text editor, notepad or somewhere else so you don’t lose it forever. WordPress will not have a backup of your edits.
“WordPress will not have a backup of your edit so FIRST copy/paste to somewhere else so you don’t lose it forever.”
Once you have safely stored your edit, you can check and fix the issue which is almost always due to a messed up .htaccess site root file.