"digital editions could not connect to the activation server"

I had the same problem and with a helpful comment about IE I found this finally worked for me:

• uninstall ALL versions of Adobe digital editions (I tried v1.7, 2 and 3!)
• use INTERNET EXPLORER to download ADE v 3. Download | Adobe Digital Editions Home
• DISABLE windows firewall BEFORE installing ADE3.
• Install ADE 3 and I found I could then aauthorize my computer using help>authorize computer in the ADE menu.
• Re-download e-book (because previous downloads had expired), and I was able to open my ebook.

I'm not at all sure it will help, but the only suggestions I have now is to try 1.7.2

Some of the bugs in ADE2 seem to suddenly appear for people when it was working before.

~~~~~~~~~

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.

http://helpx.adobe.com/digital-editions/kb/cant-install-digital-editions.html

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.