Everything in your setup looks good. The only place where I'm seeing a potential cause for that error is with Apache. The proxy we provide is written for IIS using Microsoft ASP.NET Framework 4.0. The proxy is handling signing into the group, and running this proxy under Apache is not a tested configuration.
If IIS is not an option and someone in your organization has the skills, you could play around with using something like the mono project to run ASP.NET on your Apache server, but this may also require some re-writing of the proxy/application code and would take you well beyond supported configuration of the application.
