Hi,
I understand that if you deactivate the "automatic registration" setting of JFBConnect, it works fine.
I'm not familiar with JFBConnect.
Could you explain what that setting does ?
Here is what I can say without knowning much about JFBConnect:
The Recaptcha we have was developed well before the Captcha system of Joomla existed.
It is a system plugin which implements the onBeforeStoreUser trigger of Joomla.
If the option parameter in the URL of the request is either of 'com_user','com_users','com_alpharegistration','com_ccusers','com_virtuemart','com_hikashop','com_hikamarket','com_akeebasubs'
then our plugin checks that the captcha was filled in properly. These are the extensions that are supported by the plugin and the plugin manually add the captcha to their registration page with some regex in the trigger onAfterRender.
If we could understand what is the flow of JFBConnect in the case it doesn't work, we could probably come up with some modification to avoid that situation. Maybe JFBconnect support will also have some indications for us as to what should be the best solution knowing what I just wrote above so I'd recommend you give them a link to this thread.