I've followed steps to upgrade my site from v6 to v7.4.0 and it got upgraded successfully. But I got unknown error like :
Server Error in '/' Application.
Object reference not set to an instance of an object.
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.NullReferenceException: Object reference not set to an instance of an object.
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
[NullReferenceException: Object reference not set to an instance of an object.]
DotNetNuke.HttpModules.UrlRewrite.BasicUrlRewriter.RewriteUrl(Object sender, EventArgs e) +2373
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +164
Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.34249