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If the cronjob is calling the cron file (/modules/servers/dnsprovider/dnsprovider_cron.php) properly, then you should see a log in the WHMCS → Module Logs (Utilities → Logs → Module Log) every time the cronjob is being run. You can also manually run the cron file in your browser by calling calling http://yourwhmcsurl/modules/servers/dnsprovider/dnsprovider_cron.php
Conversely, you may disable the verification mode and disable re-verification (Set it to 0).