Solved for my scenario: this was a reference problem. With the Fusion assembly binding protocol turned on, I discovered (recently migrated from x86 to x64) that the libraries I referenced in the web.config file had a strong name when compiling for x86, but when I recompiled them for Any CPU or x64, I did not use a strong name and therefore had null PublicKeyToken. Thus, assemblies that were actually deployed were not found by name at runtime.
Bad links / assembly binding problems / configuration file problems seem like a real .NET anchor point - there are many possibilities for crashes and burning with little or no direct feedback, little or nothing useful in the event log.
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