Admittedly I don't have a lot of experience with code access policy, but from what I understand, you have an assembly that calls SDWorx.GO.BBX.Client.Shell, or you have embedded 'SDWorx.GO.BBX.Client.Shell in your SA-protected assembly. You are probably using exception handling in the SA Project, and this is implementing an unhandled exception handler.
The error message itself seems to be new to .NET 4 because of a change in the security model. If this is a web application, you can add this line to web.config under System.Web:<trust legacyCasModel = "True" level="Full" />. If this is not a web application, please let me know.