[Plugin: Really Simple CAPTCHA] Trouble with multiple domains

I have a multilingual site with WPML.

Really Simple Captcha on Contact Form 7 works perfectly on default language which is on http://www.domain1.com

However, for other languages, which are on
http://www.domain2.com
de.domain2.com
es.domain2.com
it.domain2.com
etc
the captcha image is broken.

http://wordpress.org/extend/plugins/really-simple-captcha/