Server Error in '/' Application. Object reference not set to an instance of an object.

hello all,

when i try to login to the administration area of nopcommerce I get this error

Server Error in '/' Application.
Object reference not set to an instance of an object.




the actual error is



stack trace


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.

Source Error:

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.

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an object.]
   NopSolutions.NopCommerce.BusinessLogic.CustomerManagement.CustomerManager.Login(String email, String password) in C:\Users\vasilis\Desktop\nopCommerce_16_source\Libraries\Nop.BusinessLogic\Customer\CustomerManager.cs:1716
   NopSolutions.NopCommerce.BusinessLogic.Profile.StoreMembershipProvider.ValidateUser(String username, String password) in C:\Users\vasilis\Desktop\nopCommerce_16_source\Libraries\Nop.BusinessLogic\Profile\StoreMembershipProvider.cs:405
   System.Web.UI.WebControls.Login.AuthenticateUsingMembershipProvider(AuthenticateEventArgs e) +60
   System.Web.UI.WebControls.Login.OnAuthenticate(AuthenticateEventArgs e) +119
   System.Web.UI.WebControls.Login.AttemptLogin() +115
   System.Web.UI.WebControls.Login.OnBubbleEvent(Object source, EventArgs e) +101
   System.Web.UI.Control.RaiseBubbleEvent(Object source, EventArgs args) +37
   System.Web.UI.WebControls.Button.OnCommand(CommandEventArgs e) +118
   System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +166
   System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +36
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1565



end stack trace


this happened out of the blue. i have not changed anything in the site for 3 months.

when i try to add a product to a cart i get the following error

"Object reference not set to an instance of an object.  



please advise. i use 1.6 nopcommerce

anyone?

please help....i have installed a brand new version of 1.6 in my laptop. i downloaded the database from the server. i changed the connection string in the connections.config. all worked fine locally.

what could it be? is it 1.6 based on 3.5 version of .Net right? what if my host provider upgraded to 4.0 version of .net?

should I make changes to web.config?

what should be the settings in the application pool?

thanks