Hello.
At first, please read following FAQ article:
http://devnet.kentico.com/FAQs/General.aspx#FAQ96 .
It deals with similar error (the same error code) and states, that if the source of this error is located in c:\WINDOWS\microsoft.net\Framework\v2.0.50727\Temporary ASP.NET Files\, it is probably (in 90%) needed to clear the .NET cache in location mentioned above.
This issue may happen if the application is often restarted/recycled or during development when it is re-compiled frequently. This leads that sometimes the data are not loaded correctly.
Also please note this is rather issue with ASP.NET itself. It may also happen that ASP.NET inserts invalid record to .NET cache after restart. This causes this issue. You can try to convert your site to web application. As for some general info about the conversion process, you may also find useful following post:
weblogs.asp.net/meligy/archive/2008/08/03/converting-vs-2008-website-to-web-application.aspx.
Best Regards,
Radek Macalik