I am also experiencing the same 3253 error, but only in conjunction with an external HD. When I download the movie to the internal iMac HD, all is well.
Here is a screen capture of the error message.
I have attempted to download multiple movie purchases, with the same results. I also temporarily turned off my hardware and software firewalls, and rebooted all of my network hardware. The iMac is connected to the internet by an internal gigabit ethernet network. None of the older troubleshooting suggestions helped. The issue only occurs when downloading the purchased movie content to an external Western Digital USB 3.0 hard drive.
Does anyone have any suggestions on how to resolve this particular issue? I don't have the time to download the video content to the internal HD and then copy it over to the external.