macOS Sierra crashes to lock screen when playing videos on fullscreen

First, check to see if you can create a test account, to determine if it just occurs with your account or throughout the system. If it just occurs with your account (test account works normally), you probably have the same problem as the rest of us. I've been working on this for a couple of weeks with Apple Support and posted about this on another thread you posted to. I hope to hear something from Apple, based on logs I sent them after we had tried numerous other fixes, this week. If so, I will post it.

I'm having the exact same issue. When I log back in everything loads up and Chrome goes to full screen. It happens on pretty much any site I've used with videos.

If the problem is limited to Chrome, some have reported, originally on Reddit, that turning off hardware acceleration in the Advanced Settings (within Chrome) solves the problem. If you are having a more general problem with fullscreen or space changing causing a crash to the login screen, I've posted a possible, somewhat workaround-ish, fix here:

Mac Os Sierra keep logging out !!

It has worked for several people. I'm posting mostly on that thread even though there are several other threads, like this one, that appear to be focused on the same, or a similar, issue.

Have same problem, but it's just happening. First time while playing League of Legends, next time while watching movie, next time during programming. I'm not using Google Chrome

Creating a new account worked for me.

I had to do this on my last update (to El Cap) for a different reason: Mail wouldn't open.

If you have most of your docs stored in iCloud, it's not a big deal to solve the problem this way. If you have lots of pics in Photos, that can take a while to download the thumbnails, and if you have a MBAir, you can expect its fan to run flat out for a while.

But on the positive side, you'll have a nice fresh account, and you can re-install your Apps as and when required.

Dhalgren, I'd still like to hear if you get anywhere with Apple!

Update: I don't claim to have found the following solution, I came across it in another thread. I'll try to find it, and express my thanks. But here it is; it worked for me...

Go to ~/Library/Preferences, then remove com.apple.universalaccess.plist. Close the window where you did that; close any other windows that were open; launch Safari, or a video, or Preview etc then enter full screen, and it should be ok again.

The solution mombasadon suggests was posted by qkoziol at this location:

Re: Mac Os Sierra keep logging out !!

Kudos to qkoziol for tenacity.

It already appears to have worked for several people and is a pretty benign thing to do. The only thing one might have to do is reset some preferences; if the fix works, I think most people would think it was totally worth it. Unfortunately, as we have seen in this thread, no one solution appears to work for everyone, suggesting that groups of us are getting to the same place by different paths.

Weirdly, with Apple Support, I had literally trashed my entire Preferences folder, including com.apple.universalaccess.plist, and it didn't fix my problem. However, I'd certainly recommend trying it to anyone with this problem because it is so benign an action. For me, after my "Little Snitch" fix, I've not had this problem again so I can't test it myself.

As for Apple, since my last conversation with them, in which a "hot fix" was promised, I haven't heard anything. I had hoped for a fix with 10.12.1 but, clearly, that has not worked for everyone and it's not mentioned in the update description. If more people have success with qkoziol's suggestion, I will pass it along to my Apple Support contact.