Doing a quick Google it seems almost all E_ACT_NOT_READY issues are on Macs.
You have already ruled out the resolution that worked for some people, using v1.7.2.
You can have 1.7.2 and 2.0.1 installed in parallel.
It might be worth checking 1.7.2 to see if it will work for books it does support.
Even if you can't use it long term, the extra information may help narrow down the problem.
Does your online bibliothek use a special vendor ID for registration;
if it uses the standard Adobe ID (as Overdrive used for most libraries does) then 1.7.2 should work fine.
The only other thing I can suggest is another stock suggestion rather than one tailored to your issue ...
Sometimes ADE gets its registration/activation confused and in a semi-authorized state.
Uninstalling and reinstalling does not help.
Unfortunately, it often then gives misleading error messages about what is wrong.
A common incorrect message informs you that the ID is already in use on another computer and cannot be reused.
This can often be resolved by completely removing any authorization using ctrl-shift-D to the Library screen on ADE (cmd-shift-D if on Mac).
Restart ADE, and then reauthorize with your (old) Adobe ID.
In extreme cases on the mac, the following extra step has helped some people. Navigate to /Users//Library/Application Support/Adobe/Digital Editions and drag the activation.dat file to the trash. If you are using 10.7, see Access hidden user library files | Mac OS 10.7 Lion. http://forums.adobe.com/thread/1265248?tstart=0