The page error you are getting is incidental to what you are trying to do. It isn't possible to rename a blueyonder email address, or for that matter any VM email address.
If you can't get to the my profile manage accounts add an account page then you will need to call in and report a broadband fault.
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AFAIK, the renaming (alias) feature for blueyonder.co.uk email accounts has been defunct for sometime now and VM will probably advise you create a virginmedia.com email address.
IMHO, this may be an opportune moment to consider the benefits of using an email service independent of your ISP. For example, consideration of ISP would no longer be constrained by concerns over loss of email address(es).