First of all thank you for showing interest in our plugin.
This error is very strange, we never managed to reproduce it and unfortunately it does not say much. We just released an updated version of this plugin which includes some bug fixes, maybe you want to try it.
Else I suggest you to use WebsiteDefender WordPress Security plugin. This is the latest plugin we released and includes the best of all the security plugins we own.
While hoping the above helps you solve your issue, do not hesitate to post any questions you might have regarding the plugins in the WebsiteDefender plugins official forums.