R – ASP.3.5 Error while using Enterprise Library

The answer:

I had accidentaly referenced the assemblies from the src code folder. Which means the public tokens would have = null. I re-referenced to the assemblies in C:\Program Files\Microsoft Enterprise Library 5.0\Bin and the problem is now solved.

Your web.config isn't actually in the post.

In any event, you've got a reference to Entlib 2.0 in your config file. You need to update to point at the Entlib 5 version number and public key token. It's probably in your element, but if it's there I wouldn't be surprised if the rest of your file also had the wrong version #'s.

[update on further reflection]

Actually, based on the error message, there's another possibility - the runtime is finding the 2.0 assembly instead of the 5.0 assembly when it goes to load. Double check all your assembly references, and check your bin directory - make sure it's got the right assembly in it.

You can also try running the fuslogvw tool on the web server - it'll tell you what assemblies the runtime tried to load, and where it looked for them.