I've had the same issue that the everyone had and I suspect it's the same cause - that I was previously with Virgin, I called VM since Friday 20/ 05 / 2022, and tried to use my email as the sign-in email and it's generates this error code.
I've tried multiple times on multiple devices, clearing cookies, using different browsers - all that jazz.
Could an IT ticket be raised regarding this please?
Sorry to hear you are having an issue with registering for your new VM account.
Indeed, as you suspected this might be due to the same email address having been used as a username on an old account in the past.
Best way to find out if this is the case, on the login page type your email address in and hit the ''forgot my password'' button as if you were already registered, then see if the email is already in use.
Also, can you tell us which link you are trying to register on and what is the error message you get when you do this?
Please, lastly try this link here and see if this works for you as we changed it to restore issue recently?
Let us know if any of the above helped or feel free to pop back and ask for further help if you need it.