Every so often I get a call from a user saying he is trying to access some reports I wrote for him in SSRS 2005 and he gets the following error:
Cannot find or load System.EnterpriseServices.dll version=2.0.0
I’m not a .NET expert so the first thing I do is Boogle it. You will get a ton of hits on this error, but none of them talk about the solution or cause I found. Most of them tell you to copy c:windowsmicrosoft.netframeworkv2.0subversionsystem.enterpriseservices.dll to c:windowsassembly. All this does is register the missing DLL and put it back into the GAC (Global Assembly Cache). The good news is that this method can save you from having to re-install the .Net Framework. If you find that you do need to re-install, then I would suggest to first try re-applying the latest .Net Framework service pack first.
Although this may fix the issue, it did not explain why it kept happening on this system and at no defined interval. What I finally figured out is that it happens every time Windows Update has downloaded a .NET Framework Service Pack and is ready to install it. I still cannot explain how or why this would unregister the DDL from the GAC, but once I install the updates, Bingo!