Paypal Standard return error

I am getting the error below during sandbox testing. I have Testing disabled.
All works fine until the return to my site.
Any help appreciated.

Server Error in '/' Application.
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The given key was not present in the dictionary.
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.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.

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:


[KeyNotFoundException: The given key was not present in the dictionary.]
   System.ThrowHelper.ThrowKeyNotFoundException() +28
   System.Collections.Generic.Dictionary`2.get_Item(TKey key) +7455996
   NopSolutions.NopCommerce.Web.PaypalPDTHandlerPage.Page_Load(Object sender, EventArgs e) in D:\Work\Own\NopCommerce\Solution\PublishedSite\Solution\NopCommerceStore\PaypalPDTHandler.aspx.cs:66
   System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +14
   System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +35
   System.Web.UI.Control.OnLoad(EventArgs e) +99
   NopSolutions.NopCommerce.Web.BaseNopPage.OnLoad(EventArgs e) in D:\Work\Own\NopCommerce\Solution\PublishedSite\Solution\NopCommerceStore\Controls\BaseNopPage.cs:77
   System.Web.UI.Control.LoadRecursive() +50
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +627




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Version Information: Microsoft .NET Framework Version:2.0.50727.4016; ASP.NET Version:2.0.50727.4016

Sorted problem.

Could you tell us what you did to fix the error? What return url do you use for PayPal Standard?

Thanks!

See the note under step 24 here.

I can't fully remember the fix to this exact problem as I had a couple with the set up of Paypal.
But one thing that I did find was that when you are setting up a store in Euros to make sure that your Paypal account will accept euros. This is not activated as default. As it counts as additional currencies. But Safeairone has done a great step by step.