PayPal Express Setup Error: Security header is not valid

Any idea as to what might be causing this error in my setup of PayPal Express Checkout? The path in the error message is interesting, as this is not the path of the application on the web server.

LongMessage: Security header is not valid
ShortMessage: Security error
ErrorCode: 10002

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: NopSolutions.NopCommerce.Common.NopException:
LongMessage: Security header is not valid
ShortMessage: Security error
ErrorCode: 10002


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:


[NopException:
LongMessage: Security header is not valid
ShortMessage: Security error
ErrorCode: 10002
]
   NopSolutions.NopCommerce.Payment.Methods.PayPal.PayPalExpressPaymentProcessor.SetExpressCheckout(Decimal OrderTotal, String ReturnURL, String CancelURL) in C:\Users\John xxxxx.VISITUSAT\Desktop\NopCommerce\Payment\Nop.Payment.PayPal\PayPalExpressPaymentProcessor.cs:209
   NopSolutions.NopCommerce.Web.Modules.PaypalExpressButton.btnPaypalExpress_Click(Object sender, EventArgs e) in C:\Users\John xxxx.VISITUSAT\Desktop\NopCommerce\NopCommerceStore\Modules\PaypalExpressButton.ascx.cs:80
   System.Web.UI.ImageClickEventHandler.Invoke(Object sender, ImageClickEventArgs e) +0
   System.Web.UI.WebControls.ImageButton.OnClick(ImageClickEventArgs e) +98
   System.Web.UI.WebControls.ImageButton.RaisePostBackEvent(String eventArgument) +161
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +29
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2981




--------------------------------------------------------------------------------
Version Information: Microsoft .NET Framework Version:2.0.50727.3074; ASP.NET Version:2.0.50727.4049

Looks like your certificate and/or username & password may be incorrect.

http://paypal.lithium.com/pdn/board/message?board.id=wppro&thread.id=856&page=1

Thanks. I'll see if this helps and report back.

JK

I have the same problem, did you find solution?

Oh Well I set up the wrong password for my PayPal sandbox account
it is working fine now in the testing env.

How did you change the password?

i think xonix is refering to the password stored  in nopcommerce -->admin  configuration-->payment methods-->paypal

My password is correct and my key etc is correct as well. My paypal account works just fine. What is causing this error?

My username, password and signature are all correct and are from live account, still getting this error.

LongMessage: Security header is not valid ShortMessage: Security error ErrorCode: 10002

Hi Guys,
had this problem myself.

What you need to do is in your account if you're testing and have "sandbox" checked.. you need to log in to paypal and generate a sandbox api account for the merchant and a fake buyer.. from here you enter the sandbox api details and you then login as the "sandbox" customer when in the paypal side of checkout to fake purchase your goods..


your live api details won't work for sandbox testing.. if you want step by step instructions reply to this message and i can upload a doc with screenshots or something!