Solution
Sign in with your Adobe account or create a new one. As an alternative, visit the Download a free trial of Acrobat XI Pro page.
This step is required to set the session cookies on your browser which grant you access to the trial(s).
Download the official Production Premium CS5 trial directly from Adobe:
http://trials2.adobe.com/AdobeProducts/STVD/CS5/win32/ProductionPremium_CS5_LS7.7zExtract the content of the
.7z
archive using 7-Zip.Install the program using your serial number. When everything is up and running, make a backup of the setup package.