Thanks again Jason,
I did go back and double check, the check box for Test Mode is clear, and the check box for the sand box is clear. I did again copy and paste all of the credentials, and there is for sure no leading or trailing spaces in the credential text boxes.
However you mention about "make sure the billing agreement has been excepted.", sorry but what billing agreement, I am never presented with a billing agreement for this account so I seemed to be lost here.
As a last resort I will request new API credentials, but I am still a little perplexed on the billing agreement thing, could you please expand on that a little more.
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.