Error 'Timer already cancelled' when using CFIMAGE - captcha

I've implemented captcha image using cfimage. Works fine on our development system however, I'm getting a 'java.lang.IllegalStateException: Timer already cancelled' in our production environment.
The first time I requested the page containing the captcha image it took forever to load but eventially did. Susequent requests to this page now produces the above error. Production enviroment is running Coldfusion 8.01 in multiserver configuration with latest hotfix applied, IIS 6.