Spammers bypassing reCAPTCHA and log simply says "Unknown error"

Spammers seem to be bypassing reCAPTCHA, posting up to 50 comments a day.

The log shows entries of severity "error", message "Unknown error" and of type "reCAPTCHA web service" between every few comments posted.

The site has the "Omit challenges on all forms once the user successfully responds to any challenge on the site" option enabled. But every single comment seems to be from a different IP address; all anonymous users. So it seems the spammers åre bypassing reCAPTCHA,