Make sure you are using the correct settings to connect to your Mysql back-end. You also might want to check if the IP address from your WHMCS installation is allowed to connect to specific database and tables on the Mysql server.

Check: Utilities -> Logs -> Activity log, to see what is causing the unable to connect to the database error.

Search for specific parts of the code like ‘clientarea.php?action=domaindns’ en replace the matching part.

Make sure you have setup the access rules correctly under Setup > Addon modules, scroll to SoluteDNS and click ‘Configure’.