Adobe DRM: "The vendor account you entered is not associated with the item you are trying to open."

[#1]

I bought an eBook in 2015.

I cannot open it now, getting the message: "The vendor account you entered is not associated with the item you are trying to open." This is simply untrue. I've never changed by Adobe password. I moved from one Mac to another, but that shouldn't matter given the file is linked to the account rather than the device.

DRM problems again.

Please if there's anything you can do; Any help would be greatly appreciated.