Update: It happened again.  I've found that this only happens if I Publish through VS on my Production server, but not on my dev server.  It must have something to do with the configuration of some element in VS since it happens in debug through VS and in the published version.

This just started for me today.  My application had been in production and working perfectly for 6 months and then suddenly I got this error.  I tried reverting my code to older versions and it still showed the error.  In the end I removed all files from my application folder, and did a complete restore from the repo, and that got it working again.  I have no idea what caused it, but completely restoring the app from the repo was the only thing that worked for me.  I suspect the root of the issue may lie with Visual Studio.