Error passing header to paypal after upgrade from 3.5 to 3.6

Hello,

I installed 3.6 from 3.5 and everything worked great. I tried to test the paypal express payment plugin and now I get this error in my log file

The level of log entry.Log level:  Error  
The log entry message.Short message:  Error passing cart to PayPal  

The details for the log entry.Full message:  10002: Security header is not valid  

Thanks in advance