When plugging a second external monitor, macOS Sierra "logs out" (crashes?) user session. I get presented with the login window. No error message.

It's not just "show login window", because all apps load anew after logging in (which takes a very loooong time, killing my productivity).

macOS Sierra 10.12.3 (16D32).

DisplayPort external monitors.

MacBook Pro Retina Late 2013, 13".

I don't recall this happening on El Capitan. I upgraded to sierra yesterday.

There is no error message, like "was not closed normally", therefore I believe this is not a duplicate of After Installing El Capitan I Will be Logged Out Frequently (and mine is about Sierra).

I do not see anything interesting in Console logs (although maybe someone could point me to where to look).

What could be causing this? How to prevent this from happening?

What diagnostics can I do?

Edit: Some other considerations:

  1. I was fiddling with SwitchResX for custom display modes (HiDPI on external displays). As described in: 1280x720 HiDPI resolution for 2560x1440 monitor

  2. there is no corrupted graphics like in the other question(s)

Edit 2: The main monitor is: LG 27UD88-W Monitor 27" UHD 4K USB-C USB 3.0, connected via DisplayPort

Edit 3: The problem also happens when I unplug the monitor (or switch the monitor, in its own menu, to use another input cable)

Edit 4: Also posted on Apple support: https://discussions.apple.com/message/31686056

Edit 5: Also uninstalled SwitchResX (via their menu). Problem still happens!

  • Please, anyone? Very often, when I unplug my external monitor, the whole session crashes. This is killing my productivity. – KarolDepka Apr 26 '17 at 15:23
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    Try in Safe Mode. I like to begin diagnostics by starting at the bare minimum. If the problem goes away, it will most likely be something that is not part of macOS (3rd party). If it happens in Safe Mode, then it's an issue with the OS itself. If that's the case, try installing an booting from a clean install (an external drive will suffice since this is for diagnostic purposes.) – Allan Apr 29 '17 at 16:11
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    Did you ever get a solution to this problem? I am having the exact same issue. I have two external monitors (both LG - one 24" 1080p and one 27" 4K) and as soon as I disconnect them I am logged out. I usually disconnect them together - it is really annoying! – keithm Jan 9 '18 at 7:30
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    Just popping in to say I also have this issue with two displays. One an Apple Thunderbolt Display, the other a mid-2011 iMac through target display mode. Connecting either one appears to cause WindowServer to crash. There is some brief graphic distortion, displays go black and then the login screen appears. I've yet to find any workaround, let alone a complete solution. I've submitted a bug report to Apple as well. However, they've never replied to any I've submitted before so all I can do is hope for an update that fixes it. Sorry that this isn't an answer but this is my first post on here an – Doug C. Hardester May 11 '18 at 22:49
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    I've been having this same issue for a long while now. Sometimes the logout will also happen arbitrarily while the two external monitors (a Dell U2711 and an LG L1930SQ) are plugged in. I have SwitchResX running as well, so that the resolution on the Dell isn't blurry. Walked through a lot of options with Apple Support. No luck. I was hoping the upgrade to OS X Mojave would fix it, but no. – Rick Gladwin Oct 22 '18 at 19:17

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