Yes, Last Pass, Click-to-Plugin and Ad Block. I've disabled them all and I'll see if that has an effect on YouTube.
I disabled each one, played a YouTube video and restarted the extensions one at a time. All appears to be OK now--videos play as always.
Thanks for your help!!!
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Tried to play YouTube again, no go; so I disabled Click-to-Plugin and YouTube video play just fine.
Conclusion: Disable Click-to-Plugin until bug is remedied.