In my case, rebuilding my computer has not solved this problem. On my 2017 iMac 4K, I have not been able to play purchased iTunes videos since mid June. Each time I play a video, I just get a black window with the playback controls, but the video doesn't play.
I have been working with very helpful Apple support staff since June 27, but we still haven't been able to solve the problem. In my case, even a clean install doesn't solve the problem.
1) If I erase my SSD and install macOS (10.12.6, 10.13.3 or 10.13.6) via a USB installer drive, the videos will still not play.
2) If I erase my SSD and install macOS (10.13.6) via Internet Recovery or the Recovery Partition, my videos will play immediately after authorizing iTunes but only until the iMac is restarted. If I just switch user accounts after authorizing iTunes, the videos will continue to play when I switch back to the original account. If I restart the computer, however, the videos will no longer play.
3) Each time I perform this test after a clean install, I don't install any other software, so there can't be any third party conflicts. Sometimes I don't even sign into my iCloud account. I just sign into the iTunes Store and authorize iTunes.
4) If I upgrade to iTunes 12.8 after testing a clean install with iTunes 12.7, this videos will play only if I haven't restarted the iMac.
5) ****
6) Sometimes, installing Boot Camp on the iMac will cause the videos to play correctly the next time I boot into macOS, but, once again, if I restart macOS, the videos will no longer play.
7) ****
8) Strangely, my purchased videos will start playing again if I purchase a new video. Once again, however, as soon as I restart the computer after that purchase, no purchased videos (even the new one) will play. I have been able to reproduce this three times in the past month.
9) I have observed the same behaviour whether I play local copies of the videos or stream from iCloud.
10) The videos play on my iPhone 6 Plus, Apple TV 4 and Apple TV 4K when streamed from iCloud. They also play through Home Sharing with my iMac even if the iMac won't play the videos on its internal display.
11) Reseting the PRAM and SMC have had no effect. Booting into safe mode also has not helped.
12) Family Sharing members are able to play my purchased videos both locally and through iCloud streaming on various iPads, iPhones, Apple TVs and iMacs. Unlike mine, their iMacs are 2011 and 2013 models.
13) At first I thought that this was an HDCP issue, but standard definition versions of my purchased videos also have the same problem.
I'll continue to test and report back here if I learn anything new.
[Edited by Host]