I solved this on my own. My public site was set to .NET v3.5. Other integration methods specify to use 3.5 and not 4.0, so my original environment was this way. Direct Post Method works only with .NET 4.0.
I cannot explain why this setting would create the 403 error, but who cares. Swtiching to 4.0 is the fix.
01-04-2012 12:43 PM