vaasconobre on "[Plugin: PayPal Express Checkout] Get ERROR"

Hi,

My theme is "Distinction" I had activate Plugin using GBP currency work with sandbox and when apply for make a payment i get this error

Array ( [TIMESTAMP] => 2013-01-15T18:19:48Z [CORRELATIONID] => 7f0ab9a675d79 [ACK] => Failure [VERSION] => 72.0 [BUILD] => 4181146 [L_ERRORCODE0] => 10002 [L_SHORTMESSAGE0] => Security error [L_LONGMESSAGE0] => Security header is not valid [L_SEVERITYCODE0] => Error )

Any idea about this
Many thanks

http://wordpress.org/extend/plugins/paypal-express-checkout/