Hi Naomi,
When I enabled debugger I have noticed that the error originates from webuivalidation.js java script located in the aspnet_client folder within my project. Since I am almost positive that ASP.NET 2.0 does not use this script anymore since the java script functions are now embedded into .NET Framework I deleted this folder and run aspnet_regiis.
That did the trick.
Thanks again for you help.
Daniel