Security header is not valid

[Florent MANIN]

Good evening,

I'm facing some problems using the activemerchant gem (1.21.0). 
As seen in the Ryan Bates' RailsCasts (#144/#145), I would like to process payments from my app to my Paypal Account.

Everytime I try a .purchase, I got the same error: "Security header is not valid".

Ive been trying it with the credentials of two different accounts: 

• My personal French Paypal Account (real account)
• A test Account I created from the developper sandbox page (US account)
  

Same error for both accounts.

I didn't find anything usefull on Google... and already tried tones of things... but with no success :(

Any idea?

Many thanks for your time,

Florent