I need to do agent authorisation process, but I get the error "You cannot use this service"

Backstory: A previous accountant removed my business from VAT against my requests, so I fired them! My new accountant has re-registered my business, getting a new VAT number, and sent me a link to authorize them. Yet, whenever I use it I get the error "This authorisation request has already been responded to" (This is wrong, since the web page shows I have no agents authorized.) So, instead, I try and remove to "remove VAT" as a service - wait 24 hours - then "add VAT" as a service, But whatever I try I get the titluar error - "You cannot use this service" How do I use the service, so my accountant can file digitally?

Hi,
You will need to contact our VAT online services team who will be able to resolve this for you.
Get help using VAT online services
Thank you.