Were you a Readmill user in the past?
I would contact the nice people over at Blurefire Productions (About Us | Bluefire Productions) and see if they can help you get past this. It looks like since Readmill has gone out of business their website/servers have also gone down and are no longer able to authenticate valid users.
Did you join your Adobe ID to your Readmill account?
I too am suffering from this problem because I joined my Book People Vendor ID and my Readmill Vendor ID. There is clearly a request to a readmill server that no longer exists so you will no longer be able to use the adobe ID that you are trying to use. Not the end of the world, unless like me you have bought and paid for more than 100 ebooks that are associated with that ID - you will not be able to read them anymore (unless you have an older device that was authenticated prior to Readmill taking the server down).
I'd really like for Adobe to provide some support so that my eBooks can be access on my new device, but it is impossible to get somebody at Adobe that understands how all of this stuff fits together and what a Vendor ID is to reset / unjoin it. Until they do, then any eBooks you've bought are at risk, or as per my case, inaccessible.
Given that Adobe have positioned themselves as the defacto DRM standard (outside of Kindle format and Amazon proprietary DRM), I think they have a duty to resolve this issue as it is likely to occur again, and again when eBook sellers shut up shop.