RE: Customize the Unsubscribe Error Page
Hi FlowerPi,
Can you find a parameter called msdynunsubscribeid in URL when copying the "unsubscribe" link and open it in another browser?

If the parameter exists, the form should display and be prefilled with information of contact, I copied the link and opened it in both normal and private mode and the form works in both of modes.
(Or did the non-default browser block the cookies with some plugin or extension?)
The error from your screenshot indicates that there isn't a ID parameter to identity contact, thus the form couldn't display and if we show it anyway, all submissions will fail.
"We're unable to verify..." is not customizable because it has been defined by form-loader.js, and the error occurs before the form is loaded, hence the error couldn't be detected and get redirection with marketing form API.
Instead, we are only able to customize the error message when submission fails.

However, we can still use custom javascript to achieve detection, by querying whether there is a parameter called msdynunsubscribeid in URL and whether it contains value, if not, redirect page with location.href.
Regards,
Clofly