From WHMCS Documentation
The gateway log displays information about transactions that WHMCS makes or attempts with integrated payment gateways. You can use this information to troubleshoot various payment gateway issues.
You can view this log at Billing > Gateway Log.
Using the Gateway Log
If a WHMCS-integrated payment gateway has made a payment return, details of the return will appear here. Each return has three possible outcomes: Successful, Invalid, or Error.
- A successful payment return occurs when WHMCS automatically identifies and restores the payment and performs any required actions.
- An invalid return occurs when WHMCS doesn't expect the data that returns from the gateway (for example, the invoice ID is wrong or the payment value doesn't match).
- An error return occurs if the data from the payment gateway contains errors or the system can't verify it.
Invalid returns will occur when the received data is not fully valid. These are usually only random single issues. If you see a large number of invalid or error returns, contact WHMCS technical support for assistance.