Just today someone (thank you) pointed out there was an error with the recaptcha plugin in my WordPress comments. Checking it out, that “Input error: Invalid referer” is a generic error noone has ever pointed out to a single cause.
At first I made sure the keys used by the reCAPTCHA plugin were correct, and created a brand new website domain in the reCAPTCHA admin panel, and updated the values in the plugin settings. That way, I could see the captcha image correctly when loaded while being logged in, but as soon as I logged out the error was back there. I checkd the page source, and I noticed the “challenge” value of the HTML line that called the external javascript was different from the public key I updated in the plugin settings.
After some thinking, it turns out that the WP Super Cache plugin in some cases interferes with reCAPTCHA plugin, as it caches the old plublic key value which of course makes reCAPTCHA mad. Clearing the cache, thus reloading the whole page, solved the problem, and the captcha image was back there to be solved.
The problem still persists, having reCaptcha and Wp supercache doesn’t seem to be a valid option. Too bad, since I like both of these plugins and the error doesn’t always happen…but sometimes is still too much in my opinion.