Cpanel.net - unable to login anymore -bad username-

Can someone please explain/assist why suddenly logins to cpanel.net are not working with some accounts? I've had the same cpanel license account for years and was able to login as recently as a few weeks ago to sign up a new server / update licenses. Yesterday I attempt to log into "cpanel.net" to manage my licenses and get told 'BAD USERNAME' I've lodged a cpanel support ticket, which was absolutely pointless as the staff are requesting access to one of my servers to 'assist' me, when the server has nothing to do with the cpanel.net login/authentication issue. Should be noted - the email address used has the + character in it, which I use for all my services to identify who is spamming me form where etc, ie blah+cpanel@domain.com, and this worked fine until recently. The account is valid, because I'm receiving invoices and account notifications to it, I was also able to 'reset my password' thinking that might be the issue. However I'm completely unable to access my account and manage the licenses. Help!! I'm out of ideas, when I get told to contact support to resolve this, yet support literally are not reading the ticket :(