I installed AF Client 2010 R2 and PI AF Client 2010 R2 patch. Every attempt to launch AF Explorer ends up with "Method not found: 'Boolean System.IdentityModel.Selectors.SecurityTokenRequirement.get_IsOptionalToken()'."
All .NET frameworks up to version 4 installed. The machine is Windows 2008 Server.
Searching the web revealed the following hotfix from Microsoft required to fix this behavior: http://support.microsoft.com/kb/976394