I resolved my issue. I searched EVERYWHERE to try & find something related/similar to my issue but couldn't find anything. All I ended up needing to do was log in with the administrator@vsphere.local acct > Administration > Configuration > Identity Sources tab, then simply remove my domain and re-add it as "Active Directory (Integrated)". I can now log in with my account to the Web Client. I was able to log in to the C# client ok, but was having issues with the Web Client. Hopefully this helps someone in the future.
Regards,
Shane
(twitter: @coolsport00)