Vendor ID Authorization Issue

I purchased an eBook from Broadview Press. I want to read that eBook on Adobe Digital Editions. However, whenever I attempt to do so, Adobe Digital Editions requests a Vendor ID. My login, which is correct, does not work.

Specifically, the "error" I get is:

 Then when I click "Continue"...

 I have an Adobe account, but logging in just results in this error:
"The vendor account you entered is not associated with the item you are trying to open. Try again."

Which makes sense, because I'm not a vendor.

What I don't understand is that I have opened, viewed, and read this book before. Many times. It's just suddenly, today, it doesn't work.

I have tried:

• Contacting Support. They sent me here.
• Unauthorizing and re-authorizing my ADE.

I'm stuck. Help, please.