I don't think that the Kobo account is relevant at all.
Also, email address capitalization is usually ignored; though maybe there is some bit of ADE (Adobe Digital Editions) or the Kobo that does not ignore it.
When you say my (Adobe authorized) Kobo Mini reader I assume you mean that you authorized it using 'Authorize Device' from ADE?
It might be worth trying ADE1.7.2, though I am not at all sure it will help.
ADE 2.0 has some peculiar bugs: for example it won't work at all with most Sony eReaders for reading library books.
Yours is clearly a little better than that, in that it works enough to get the book to the eReader, which is why I am a little pessimistic.
However, using 1.7.2 has proved a success for many different issues with 2.0 on this forum, so worth a go.
There are lots of bugs in ADE2.0 (and 2.0.1). Try replacing ADE2.0 with the older but more reliable v1.7.2.
(You can have them both installed at once if you like.)
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172
Some people have found ADE trying to upgrade automatically.
It appears (not 100% sure) that if you install ADE2.0 as a new install (not as an upgrade) that your 1.7.2 will continue to run.
Probably best to say no if 2.0.x installation asks if you want to migrate your library.
Thank you for your help sjpt, but no luck with ADE 1.7.2. Same problem.
I may try again starting from scratch... maybe with another account... maybe after calling Kobo... or will simply avoid DRM-protected content on the reader until all stakeholders figure out how to better integrate the various bits and pieces for a seamless end-user experience.