The Adobe DRM team are the experts at taking something that needs to be simple for a non-computer person to use, and making it so that it is almost impossible to use even for someone really experienced. 'ADEPT' seems the most inapt name for the Adobe DRM mechanism. Adobe Digital Experience Protection Technology: they want to protect you from the experience of actually reading your digital book?
There is a horrible suggestion for E_ADEPT_INTERNAL at http://helpx.adobe.com/digital-editions/kb/error-e-adept-internal-down load.html
I think I'd wait to see if the bookshop can help first before trying it, though.