SSO connection to Plesk though WHMCS fails. Login to Plesk from WHMCS redirects to Plesk login page back instead of logging in.
WHMCS is behind Cloudflare,
User's IP address is changed during a Plesk session. This may happen if user's IP address is dynamic or the domain/hostname which is used as a URL for Plesk is behind Cloudflare.
By default, Plesk does not allow IP address changes during a single session.
Allow IP address changes during a single session:
Log in to Plesk (use the server IP address if it is not possible to log in using the hostname).
Go to Tools & Settings > Session Idle Time and enable the option Allow IP address changes during a single session.
Click OK to apply the changes.