Good approach to troubleshooting, 99jon,
Yes, they all play in Lr CC on the web.
And, as I've described, on iMac- ... and yet not iMac-2.
And, oddly, on iMac-2, one of the videos plays, while the other three do not!
So there's some issue with iMac-2 that is otherwise running the same OS and Lr CC as iMac-1.
I'm wondering if there are perhaps preferences files that could be deleted?
Or perhaps I should delete Lr CC on iMac-2 and reinstall?
Thought I should continue to investigate:
I just de-installed Lr CC and chose to not save the preferences.
Re-installed Lr CC ... and the same three videos that play on iMac-1 and on my Lr CC online/web app still do not play on iMac-2.
I realized that in Lr CC Preferences>Local Storage>Options, I have checked "Store a copy of all originals at the specified location," so these videos should also be on my hard disk. I located them at the Finder level, and they play both there and when opened in QuickTime, so it seems like the issue has now been pretty much isolated to playback within Lr CC that is not working, but only on iMac-2.
Actually, I just realized that almost all videos in Lr CC on iMac-2 fail to play ...