This issue unfortunately pops up occasionally and is generally fixed by performing the following actions:

Reissue the license:

Proceed to your WHMCS client area and login (http://www.whmcs.com/members/index.php). Your login details are generally the same as your account here with LicensePal unless the password has been changed since signing up. Once you are logged in navigate to the My Products/Services area of the WHMCS portal. From here you will view the details of the license in question. Towards the middle of the page you will have an option to reissue the license. Once the license is reissued proceed back to your WHMCS installation and try once more. Note: You may be prompted for your license key prior to logging in.

If reissuing the license does not rectify the issue then please contact the WHMCS support team for further assistance.