Hmm, I think I know why you see this error. There is already been made a My Ziggo account for your new subscription (with a different e-mailadres). So currently you have seperate credentials for you new My Ziggo account and My Vodafone account. Can you try the following steps to extend them?
My Ziggo:
Log in to My Ziggo
Go to details (gegevens)
Scroll down until you see 'Inloggegevens Mijn Ziggo'
Click on 'Ga naar Mijn account'
Click underneath 'Wel al een My Vodafone account' on 'Accounts samenvoegen'.
Click on 'Start samenvoegen' en log in with your My Vodafone credentials or use the QR scanner in de My Vodafone App
Attention: The login credentials of your My Vodafone account become also the login credentials for My Ziggo.
You can also do this in My Vodafone.
Log in to My Vodafone
Go to your profile 'Profiel'
Click below 'My Vodafone Account' on 'Account instellen'.
Click underneath 'Wel al een Mijn Ziggo account' on 'Accounts samenvoegen'.
Click on 'Start samenvoegen' en log in with your My Ziggo credentials
Click on 'Nu inloggen' and the accounts will be extended.
Attention: The login credentials of your My Vodafone account become also the login credentials for My Ziggo.