ReCaptcha suddenly shows only one challenge-word instead of two

Without any changes to the site, the ReCaptcha suddenly shows only one challenge-word, i.e. the "unknown" word. As the one word known to ReCapthca is not shown, the form has been made totally useless.Has anyone any idea how to fix this? -- You received this message because you are subscribed to the Google Groups "reCAPTCHA" group. To unsubscribe from this group and stop receiving emails from it, send an email to recaptcha+unsubscribe@googlegroups.com. To post to this group, send email to recaptcha@googlegroups.com. Visit this group at http://bit.ly/1dkFnYd. For more options, visit http://bit.ly/P65DvS.