Quite tricky to give a reply without having access to your solution but I would start investigating the following:
Are all your EPiServer assemblies in the bin-folder correct version?
Does your Visual Studio solution refer to the correct versions? (adding EPiServer through Nuget use to be the simplest way to keep track on versions)
Does your web.config have any assembly bindings that lead to the wrong versions?
Thanks for your reply.I have upgraded to 7 when upgrading to 7.5 in the step Install EPiServer.CMS.Core i am getting error saying that Install failed. Rolling back
Failed to add reference to System.core. Please make sure that it is the GAC. Any idea how could i resolve this.
System.Core is a .NET assembly so make sure you have installed the correct version of .NET framework on you dev machine. Check under project properties which version the EPiServer solution uses (or in web.config)
Also turn on full log4net logging for EPiServer to give you more information about what fails...
Server Error in '/' Application.Could not load type 'EPiServer.Shell.InitializationModule' from assembly 'EPiServer.Shell, Version=6.2.267.1, Culture=neutral, PublicKeyToken=8fe83dea738b45b7'.Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.
Exception Details: System.TypeLoadException: Could not load type 'EPiServer.Shell.InitializationModule' from assembly 'EPiServer.Shell, Version=6.2.267.1, Culture=neutral, PublicKeyToken=8fe83dea738b45b7'.
Source Error:
Source File: c:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root\0a09daa4\2d581c07\App_global.asax.0.cs Line: 0
Stack Trace: