Domain Mapping returning error 'invalid IP' from WHMCS

Hi there. I have installed WHMCS on my server and I have configured domain mapping with my dedicated IP address which has been added to the IP whitelist in the WHMCS admin area, yet when I go to configure my reseller account in the reseller section of the domain mapping plugin I am being returned an error saying 'Looks like your credentials are invalid. Please, check the errors sent by WHMCS server: Invalid IP'. I tried the support forums but couldn't find anything that solved this problem. I'm a bit stuck now. Your help would be appreciated. Thanks.