Invalid JSON issue with CF7

Hi there – been using CF7 on our clients sites for years with no issues, and all of a sudden, on multiple sites, we are getting the “The response is not a valid JSON response” when we try to update or create a new page / post etc.. and this morning I am seeing a site scan issue on another client site and the message “Invalid JSON.” causing the site scan to fail.

Like a good WP user that I am, I have checked all the usual suspects (URL’s – all correct, Permalinks – all correct, .htaccess file – all correct etc..) and then did the usual ‘deactivate plugins – turn back on it turn’ routine to find the culprit… and it was CF7..??!

I have had to install A N Other form plugin.. and everything is fine and dandy with no issues. What’s happened recently (i.e. in the past few days, week at most) to CF7 to cause this.