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Youtube with ClickToPlugin extension (Safari's plain, default HTML5 video player this is) plays just fine. Flash player also works.

Taking into account all previous comments (changing agent makes it work, also happens on 10.10) this is clearly YouTube's fault.
 
I'm not experiencing any issues with YouTube in Safari on either my 13" MBP, 15" rMBP, nor my 27" iMac.......all running Yosemite 10.10.1
 
I'm starting to really hate Safari. It's become really slow and laggy!! I would completely switch to Chrome but Chrome lacks Safari's smooth zooming and I use that a lot...
 
I'm dying at all the people in here screaming at Apple. Thanks for the :D.
 
Can't watch any YouTube videos. Confronted with a blank, black box and the video won't start playing. I also get freezing and the beach ball while this is happening.

I imagine this is not an error on Apple's part.
 
I'm starting to really hate Safari. It's become really slow and laggy!! I would completely switch to Chrome but Chrome lacks Safari's smooth zooming and I use that a lot...

Trollololol. Wait, so Safari is too laggy for you, so you want to switch to Chrome, but Chrome lack's Safari's smoothness? What?
 
It seems to me this is almost certainly an issue on YouTube's end, not with OS X or Safari, given a few things that have been mentioned:

1 - The issue started today, not yesterday. If the issue was introduced in the new Safari, it would have started yesterday, when it was released, not today.
2 - The issue is also coming up for people who didn't update to 10.1 yet - people still using 10.0 are also getting the issue. So it definitely isn't caused by the update from yesterday.
3 - Changing the browser agent fixes the problem. All that does is change YouTube's server to treat your browser as if it were a different browser. This doesn't actually change your browser.

So all signs point to YouTube having an issue with processing the agent string of Safari 8. Perhaps they maintain a whitelist or blacklist for which browsers should be able to access YouTube videos, and they accidentally removed Safari 8 from the whitelist (or added to the blacklist)... or perhaps they did it on purpose, to spite Apple and Apple's users. It wouldn't be the first time Google attacked Safari - remember when they exploited bugs in Safari's cookie policy so that they could track Safari user's via their ad network even when users explicitly asked not to be tracked?

Could it have something to do with the new flash update. Thats the most recent software change I have made to my system.
 
Can confirm. With the move to HTML5 as well, Adblock for Safari 8.0 is absolute garbage on Youtube and doesn't block video ads, I need to refresh multiple times.
 
I don't think this is a Yosemite problem or a Flash problem. I tried some other websites that use flash for video's and those seem to work properly with no errors or audio/video delay. So I really think the problem is on YouTube's end.
 
Thank god I'm not alone! I have spent the last few hours trying to find a solution... I was even contemplating doing a fresh install of Yosemite!
 
I'm dying at all the people in here screaming at Apple. Thanks for the :D.

Sure, it's a knee-jerk reaction, but since it works in Firefox and Chrome..., even if Google did change something (which I'm sure they did), it's odd only Safari doesn't work.
 
I'm dying at all the people in here screaming at Apple. Thanks for the :D.

A reasonable person would assess the issue, realize the potential that either involved party could be at fault, and hold judgment until further evidence arises.

But this is MacRumors, where everything is Apple's fault, all the time.

Yup, just adding this problem to my list of other problems with Yosemite

Exhibit A.
 
Windows too?

I had a similar problem today in Windows 7. A strange black overlay screen appears, partially covering the previous screen, followed by an error message.

First time I've ever seen anything like this. Will try on my Mac when I get home tonight.
 
The whole Yosemite is just a crap.
The same with working with vmware fusion. It slows down the whole computer and makes is unsable. Not for professional user. Maverikcs is fine and proven...and looks better. Apple joint companies like Garmin, when firmware 4.0 starts working. All before is just a beta. Savings on testers. Users are testers.
 
I had this issue before the update. It's pretty obvious that it's a YouTube problem..

I cant believe people thought this was a Safari problem when YT was the ONLY site displaying any issues.

The anti-Apple sentiment on this forum is mind-boggling sometimes.
 
Well it'll be interesting to find out the cause of this one!

I suspect it is a bug in Safari 8 - that's always existed- and Google's just enhanced their HTML5 YouTube player and now it's triggering the bug in Safari. Awaiting an update from Apple to address playback issues with certain HTML5 videos.
 
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