Well, the error being returned is:
Security header is not valid
this error occurs if you are using incorrect API information. there are 4 main cause for the proble:
1) Using Live Credential and processing through the sand box
2) using sand box credentials and processing through the live server
3) Not excepting the billing agreement after requesting the API credentials.
4) sometimes the api credentials generated by paypal are just bad.
things to check:
make sure to turn the sandbox on or off depending on the type of API credentials you are using.
Makes you are copying and pasting the API Username, password and signature into the admin section.
double check for leading or trailing spaces in the api info text boxes
make sure the billing agreement has been excepted.
if all else fails, request new api credentials.
