RESOLVED! One of the test WordPress instances had an impossible, unpleasant and not covered on Google error message. When user is trying to register, the CAPTCHA code is displayed properly and validation is correct, but when the button Register is press, the page would display this: ERROR: Unreadable CAPTCHA cookie I have a plugin “SI CAPTCHA Anti-Spam” installed. The error message is from this plugin. I’ve tried changing browsers, clear cache and cookies. Nothing worked. No luck on Internet search either. What to do? Finally I changed one of the settings on the plugin itself. I check an option for: Use CAPTCHA without PHP session And it worked! The error message…