Hi Palli, thanks again for working with me on this.

I believe I have found a resolution.  I changed the "Enable 32 Bit" flag to False on the application pool and removed the 32-bit Handler Mapping, leaving only the 64-bit Handler Mapping.  (If the "Enable 32 Bit" flag is set to true then the 64-bit Handler Mapping is ignored, which was the original problem we reported... I guess that makes sense.) Now the 64-bit assemblies in the bin folder don't cause an error. 

This is actually how we have been deploying the website for several years now.  We have about 6 versions of our site deployed to various test/QA environments with VWG 6.3.15 and they all have "Enable 32 Bit" set to false and a Handler Mapping of c:\Windows\Microsoft.NET\Framework64\v2.0.50727\aspnet_isapi.dll.  This is what finally gave me the answer.

Thanks again,
Andrew

Hi Palli,

Yep, case closed.  "Enable 32 bit" is false and the handler mapping is set to C:\Windows\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll

The site seems to be working fine.  6.4 renders out some of our controls differently, so I'll have to work through that.  But otherwise all seems well!

Thanks for helping us work through this,
Andrew