When exactly does the error occur?
Is it when you open the downloaded .acsm files in ADE to download the associated DRM .epub file,
or have you already got as far as getting a .epub file?
One possible reason is if you updated to the buggy ADE2.0.
Other people have had that issue with that horrible software release.
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