It might be issue from Firewall(CSF) and mysql allow host. Please fix it by using the steps below:
- Add Special User to billing system and try manual login to https://www.yourdomain.com/clients/clientarea.php then update new Special User-Password in WHM (root) >> RVGlobalSoft Manager >> RVSkin Manager >> Integration >> 3rd-party single sign-on system >> WHMCS.
- Upload RVSkin interface to /home/{account}/public_html/clients/rvwhmcs.php