HTML videos not playing on Sierra 10.12.6 I'm using rMBP 13" mid-2014 and I'm NOT updating to High Sierra, but even good old Sierra is not as stable as older versions of MacOS.

Sometimes I can't play videos, I press play and nothing happens, YouTube, Vimeo, Netflix, everything is down. Usually restarting browser(s) helped and normally it happened only in one browser at a time but a few minutes ago I had to restart my Mac, because every single browser I have (Chrome, Opera, Safari, Firefox) couldn't play a thing and restarting them did nothing.

Is it a known issue? How to fix that?
The first thing I'd probably do is start your Mac in Safe Mode and test to see if you can play videos fine? 

Boot into Safe Mode

Follow these steps to boot your Mac into Safe Mode:


Fully shut down your Mac
Restart your Mac
Immediately press the Shift key and keep it down 
Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice).
Take a note of what happens (i.e. can you play videos okay now?)
Exit Safe Mode by restarting your Mac as normal
Again, take a note of what happens (i.e. can you play videos okay now?)


Once you've booted into Safe Mode, let me know how you went and we'll go from there. Primarily we need to establish if everything works okay in Safe Mode or not. If so, we need to then establish whether videos can play okay when booting normally afterwards.