In case you have implemented Identity Management and see a blank Double Opt-in confirmation screen after clicking on the verification link received in your email (e.g. https://piano.io/?verification_code=DOVCFQK1ZWIZ62), you will need to add your website's URL to the list of Authorized Javascript Origins in the Identity Management configuration under Edit Business:
Please note, that depending on your setup, you might need to add your website's URL with or without a forward slash character / at the end.