Same with me, changing to [1.4] works!
( Just replace the file really-simple-captcha.php from website http://plugins.svn.wordpress.org/really-simple-captcha/tags/1.4/ )
My problematic 1.5 installation is on an IIS machine... so maybe that's the issue with 1.5?
Evan
http://www.holistic.com.mt
http://www.evancamilleri.com