iTunes update

Hmm, not sure. If QuickTime is working then at least you should still be able to play all of your iTunes movies and TV shows. Not ideal, but at least you can still access the stuff that you paid for.

I just noticed in your original post that you were on High Sierra 10.13.2 rather than 10.13.3. I wonder if iTunes 12.7.3.46 is fully compatible with 10.13.2? Both updates came out at the same time, so perhaps something in the latest iTunes was expecting the latest High Sierra version? My only reco would be to reload iTunes, as maybe your version is still looking for the decode to m4v files in the wrong place? This reco is just a shot in the dark, but iTunes is not very big (unlike 4.8GB for a full High Sierra install), so downloading and trying again is not that painful.

If that doesn't work then I would recommend contacting Apple, if you haven't done so already. I ultimately solved my issue myself, but I give them credit for throwing resources at the problem. FYI, when playing videos, my window looked like this. Is that what yours looks like?

I have the same issue, iMac (Retina 5K, 27-inch, Late 2015), macOS High Sierra (10.13.3). I reinstalled iTunes (already had 12.7.3.46, but tried again), and like ccmorz, my video is choppy, while the audio is fine. Video worked fine prior to last update of High Sierra and iTunes.

ccmorz, can you play an .mp4 video? iTunes videos are .m4v, which are basically .mp4 with DRM-encryption. In my case I think the updated iTunes and QuickTimePlayer are unable to decrypt iTunes videos, as both can still play .mp4 videos correctly.

I have a call in with Apple Support that has now been escalated up the chain on this (to the point that Apple Engineering now has an image of critical files, systems activities and a log run while I 'played' a video). I sent them a link to both your post and another post from someone else with the same issue, so any solution that they come up with for me will hopefully work for you and the other user.

There is internet software that can strip off DRM encryption - converting .m4v to .mp4 - but I'm not ready to go there yet. Another solution if you have Time Machine is to roll back the updates, but even Apple indicated that this would only work until the next update. I'm also not ready to go there at this point. My temp solution is to copy videos (cuz I downloaded them) to an iPad or my not yet updated MacBook Pro to watch them. But this is obviously not an acceptable long term solution.

All that being said, Apple does seem to be working hard on this issue, based on what I can see of the people that I am dealing with, so I do anticipate a solution within the next week or so. I'm not thrilled that the issue occurred, but I am comfortable with their response so far.

Not yet. Apple collected some more info from my iMac, including tar files generated from my /Users/Shared directory, plus data from my ifconfig, and /System/Library/PrivateFrameworks directories. They then asked me to delete a couple of directories that iTunes would auto-repopulate, thinking that was going to work, but it didn't.

I ran the same file generation commands against my MacBook Pro (which still works, but without the os or iTunes update - I held off after the iMac issue), and I noted a couple of differences in the ifconfig output (iMac 'media: autoselect (none)' and 'status: inactive' vs 'media: auto select' and 'status: active' on the MacBook Pro). Perhaps this is the issue?

So at this point I'm waiting for the engineers to figure out what to do next.

Did you try reloading the os and iTunes updates? I reloaded iTunes, but not the os update. I live in the exurbs, and my internet access is a proverbial can at the end of a long string, with less than unlimited GB each month. As such, I held off on downloading the os update (>2 GB) a second time. I asked Apple whether I should reload the os, but they told me to hold off for now. Have you tried doing the update a second time?

Okay, fixed. The last I heard from Apple in terms of "try this" was on Feb 2nd, and on contacting them on Feb 8th I heard that the issue was being handed over to the iTunes engineers. Given that today was Feb 14th, my patience expired and I decided to do a full re-install of the OS (despite Apple telling me in the past to hold off).

If you're like me you do the updates when they arrive, which I believe means that you are only doing an incremental update, i.e. the difference between 10.13.2 and 10.13.3. This time I did the full reload from the App Store -> macOS High Sierra, all 4.8GB (but 5.2GB to download - huh?). Whatever was missing in the jump from 10.13.2 to 10.13.3 was fixed in the full re-install, and now iTunes and QuickTime Player are both working properly. I would note that my iTunes app in Finder still says last modified on Jan 25, and QTP still says Dec 19th, so that suggests that a) they weren't reloaded and b) were not the problem. This makes sense if the issue was decoding the .m4v file, since my assumption is that both apps would call the same decode module rather than each having their own version.

So, your call, but for me the full reload - again, not an incremental update but instead the full Monty - worked. I'll make sure that it is still working tomorrow morning (it is now 1:12 AM here), and if so I will let Apple know. If you try this, let me know how you make out.