Currently in the Spring Security 3 support, we try to auto-detect a unique AccessDecisionManager for use by the <flex:secured-channel> implementation. An error is thrown if there are more than one AccessDecisionManager in the context, which is now common practice in version 3. Instead of trying to re-use one of the internal Spring Security instances, we should just configure one specific to the needs of <flex:secured-channel> so as to avoid conflicts.

See the referenced forum thread for an example of the problem, and for the ideal way to work around it in the meantime.