UPDATE- RESOLVED 3-21-2018:
Adobe has resolved the issue with a new version of Digital Editions (v 4.5.7.182628). This version is available for download from:
https://www.adobe.com/solutions/ebook/digital-editions/download.html
Please see below for details of the error as originally reported.

There is a widespread issue with the latest version of Adobe Digital Editions (v 4.5.7.181275), as of March 20, 2018. Users are getting the following error:

Screenshot of ADE error that reads 'unable to download'

Adobe has acknowledged the problem and is working to resolve it. In the meantime, there is a workaround, which is to uninstall the latest version of ADE and reinstall an older version available from https://www.adobe.com/solutions/ebook/digital-editions/download.html. The download version on this page has been updated to a previous version, v.4.5.7.179634. Updates are also being posted to the Adobe forum thread at https://forums.adobe.com/thread/2467374.

Although this issue affects mobile versions of Adobe Digital Editions, the issue will not affect downloading to mobile devices on Ebook Central since a different app, Bluefire Reader, is required.