https://hostcheetah.com/t/whmcs-returns-to-login-screen-after-entering-in-credentials/154

Troubleshooting Login Problems
If when entering login details you are redirected back to the login screen without any error, this indicates an issue with PHP sessions on your server.

This can be confirmed by ticking the “Remember Me” checkbox on the login page:

https://hostcheetah.com/t/whmcs-returns-to-login-screen-after-entering-in-credentials/154

If you are logged in with this option ticked, it confirmes there is a problem with PHP sessions. Common things to check for include:

The configured session tmp path not being writeable
The configured session tmp path being full
This is a PHP configuration level issue and therefore for shared or reseller users, will require the assistance of your web hosting provider.

Global Web Hosting by Liquid Layer Networks
http://www.LiquidLayer.net