v155a (or thereabouts...)

i'm trying to do some testing with paypal express. i configured the paypawpp module on the admin side. if i set it to live mode, i am directed to the paypal site as i would expect. if i set it to sandbox mode, i stay on the ZC site and i get the following error:

Code:
Response: 
Array
(
    [TIMESTAMP] => 2017-10-18T02:03:19Z
    [CORRELATIONID] => efc504ddba46f
    [ACK] => Failure
    [VERSION] => 124.0
    [BUILD] => 39972400
    [L_ERRORCODE0] => 10002
    [L_SHORTMESSAGE0] => Security error
    [L_LONGMESSAGE0] => Security header is not valid
    [L_SEVERITYCODE0] => Error
    [CURL_ERRORS] => 
)
i have run the extras curltester and gotten this response:

Code:
Connecting to PayPal Express/Pro Sandbox ...
GOOD: CURL Connection successful.
any idea how to run some tests using sandbox mode on paypal express? what might i be doing wrong? it seems that the api credentials are correct as it does work in live mode.

thanks in advance.