Client complains he paid invoice, but coinpayments not marked it Paid.
In, his payment is marked as Complete, you even go to IPN section of the coinpayments account and see it is successfull even you re-send request.

But in WHMCS, invoice is still marked Unpaid.


check your server (where WHMCS is hosted) error and access logs. (in case of cpanel, access logs are in /home/username/access-logs/ or in cPanel/Metrics/Raw Access/)

filter out log lines containing callback.php, you may find your server blocked the coinpayments robot (403 or 302 redirect to banned.php) or such. Coinpayments has IPN server returns success status just because your server returns 200 HTTP code. If returns 403 (forbidden), then IPN fail. Make sure to whitelist/ignore their IPN server IP.