Are you using any plugins that provide a CAPTCHA services? There are a lot of different anti-spam plugins that use CAPTCHAs to try to detect spammers.
As best as I can tell from the code for the Coraline theme there are no captcha functions being loaded.
If you cannot easily identify the plugin causing the issue, I would recommend disabling all of the plugins on your site, see if the issue still occurs. If it does not, re-enable your plugins one at a time until you cannot post a comment. When that happens, you'll know which plugin is at fault and you can either disable it permanently, ensure that it is up-to-date and does not require an API key or something similar, or you can post to the support forum for that specific plugin to get help with resolving the issue.