PayPal sent the transaction to your processor but didn’t receive a reply on time, or communication between your processor and PayPal terminated unexpectedly.

Modify timeout value
If we didn't receive a response from your processor within 30 seconds (the default value), you can modify the client timeout value for Payflow Pro accounts only when sending data to PayPal.

Communication terminated
Even if communication with the processor fails, an authorization can still be executed, and the customer's credit card can be temporarily charged.

• For merchants using PayPal as their processor: If you get result code 104, use your PayPal Manager account, along with your PayPal account, to try to reconcile transaction processing. Some transactions that resulted in a result code 104 may have been processed, and some may not have been processed. To help you reconcile, you may want to use our Instant Payment Notification (IPN) feature, which automatically notifies you of payments received through us. You can then search your own database for the payment received, should a result code 104 error occur. You can also include an invoice number (INVNUM) parameter in your transaction request, which would let you log in to your account and see if there was a successful transaction using that invoice number.
• For merchants using Global Payment Central, PayPal Australia, Moneris, Litle, Telecheck 2, or Planet Payment: Because these processors are host-based, the process transactions immediately. Contact your processor and your merchant bank to determine whether the transaction was received. If it wasn't received, resubmit the transaction for processing. If it was received, update your records accordingly, or perform a credit transaction and then resubmit the transaction for processing.

Note: For other processors, if an authorization exists, you can enter it as a voice authorization, or you can resubmit the transaction for processing.