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Make sure all sections are green. If there are RED or ORANGE sections, adjust your PHP and server settings.
Since all data is fetched from WHMCS in real time, caching will not work, this includes WP-Cache and CloudFlare (or any other services like that)
Double check if WP > WCOM > WHMCS Config > WHMCS URL is exactly the same as in WHMCS > Settings > General Settings > WHMCS System URL
SSL Certificate: If you are using SSL for WHMCS, Check your WHMCS URL here for SSL problems. https://www.sslshopper.com/ssl-checker.html
If you find errors, use WCOM > advance settings to ignore SSL errors.
Authentication Failure for Invoices:
The authentication key is not validated during setup but is used for viewing invoices and some other functions, make sure it is setup correctly.