"Security header is not valid" Error
This error message means that your PayPal credentials are not valid. Please make sure that your PayPal account ID, username, password, and signature are entered currently in Account > Settings > PayPal
"User is blocked" Error
This error indicates that you do not have the Mass Payments API enabled on your PayPal account.
If you're attempting to send a Mass Payment for first time you MUST contact PayPal to enable the Mass Payments API for your account. PayPal typically takes 24 hours to approve Mass Payments API requests. There may also be restrictions for your currency, country, or account.
"Insufficient funds" Error
This error occurs when your PayPal account has insufficient funds.
When sending Mass Payments via PayPal, you must have enough funds in your PayPal account balance in the respective currency in order to cover the payment(s) being made. PayPal will not automatically charge your linked card/bank account, so please be sure to fund the account before making a payment.
IMPORTANT TO NOTE
Funds in your PayPal account must be in the currency that is being paid out. For example, if all the funds in your PayPal account are in U.S dollars, and one of the payments you are sending is in British Pounds, then you will receive the above error message.
"Processing: (that never updates)" Error
This error occurs when the IPN address has not been updated in PayPal. Double check that you properly added your IPN address in the Mass Pay setup article. If this does not resolve your issue please contact Paypal.