CF code works fine on IIS6 but fails under IIS7.5 with "Complex object types cannot be converted to simple values"

CF 7 and CF 8 produce the same error in IIS7.5 on Windows 2008 R2. It works fine in IIS6 on Windows 2003.

Code for the normal http website seems fine, but when the code calls https sections of the site, I get this error. The debug error is "Complex object types cannot be converted to simple values" and the detail shows the problem occurs in fbx_fusebox30_CF50.cfm when it reaches:

     FB_.ReverseCircuitPath[fusebox.Circuits[aCircuitName]]=aCircuitName;

     if (ListLen(fusebox.Circuits[aCircuitName], "/") EQ 1){

          fusebox.HomeCircuit=fusebox.Circuits[aCircuitName];