It is very hard to diagnose this kind of problems. I have debugged many
access denied errors. And most of them boil down to the following scenario:

1. You enabled shadow copy on the AppDomain.
And,
2. You have either Anti-Virus or index service enabled.
And,
3. You may have run the applications many times in a relatively short time.

The reason is related to how fusion does shadow copy. Here is exactly what
happens.

1. Say you enabled shadow copy on the AppDomain, and you load an assembly.
Fusion will first copy the assembly to a temporary directory under the
shadow copy directory, then call MoveFile to move the temporary directory to
the final location under shadow copy directory. The reason is that we want
the shadow copy to be atomic. And MoveFile gives us the atomicity.

2. If you have Anti-Virus and index service enabled, and you are unlucky,
after we have copied your assembly to the shadow copy directory, and about
to call MoveFile, Anti-Virus or index service decides to poke around the
temporary directory. Now when we call MoveFile, we get the access denied
error.

Condition (3) is not necessary. But it greatly increases the possiblity of
the error. When you read/write the same file many many times, it will alert
the anti-virus.

The fix is easy, excluding the shadow copy directory from Anti-Virus or
index service.

How do you know if you are hitting this scenario?

First, download File Monitor from http://www.sysinternals.com. Run
filemon.exe and set the filter to only trace file accesses under shadow copy
directory. Now run your applications. If you see the Access Denied
exception, you can stop the filemon tracing. Now search the filemon log for
"ACCESS DENIED". When you find it, look at the a few lines above. If the
application on the a few lines above is Anti-Virus or index service
(cisvc.exe), you know you are hitting this problem.

If this is not your case, sorry, I don't know why. You have to debug it
yourself. And you should start with filemon.