The T2S_MentalRay addon includes copies of Softimage DLLs, such as sibase.dll, and that causes problems in Softimage 2011. I’ve had two cases of shader/workgroup problems that were resolved by removing this addon.
Update: As noted in the comments, the problem is older versions of SPDL files that contain GUIDs used by the Softimage factory shaders. Addon shaders are loaded first, so these SPDLs override the factory defaults and cause problems.