Announcing the release of a Critical security fix for .Net 4.0 and an update for that fix.  On July 9th, 2013 there was a Critical Security Patch (KB2840628) that was release to mitigate some identified security risks with .Net 4.0.  This critical update caused an exception with sand-boxed plug-ins when using of specific methods within the System.Diagnostics namespace.  This .Net team has identified the source of the problem and has issued a patch to address this problem.  The patch is posted in this article:

If you believe you are encountering this exception and are using plug-ins registered in sand-box mode, the error message below is the primary indicator you are running into this problem. This will also only occur for .Net Framework 4.0, there has been no reproduction of the issue with .Net Framework 4.5.

System.MethodAccessException: Attempt by method ‘System.Configuration.TypeUtil.CreateInstanceRestricted(System.Type, System.Type)’ to access method ‘System.Diagnostics.AssertSection..ctor()’ failed.

If you are still encountering this message after applying the update please open a support case so that we can troubleshoot further.