ADE fails to recognise account credentials

I'm trying to authorise a newly built Windows 10 PC with my Adobe ID. My login credentials (email, password) work fine for logging into the Adobe website, but are rejected by ADE with the error message "Incorrect login ID or password for the selected eBook vendor. Please try again."

"Adobe ID" is selected in the eBook Vendor drop-down; I've never used any other. Adobe support were stumped and referred me here. Any help or advice welcome. TIA.