Some users that have a Intel 3rd Gen Core processor Graphics Controller on their computer, possibly have experienced a bug on Linux Mint 19 when browsing folders with Nemo, on that just by opening a folder containing videos, it instantly logoff the user.
It occurs because Cinnamon and this kind of graphics processor chipset "does not talk very well" (reason why it normally starts in software mode, without graphics accelleration) and Nemo uses part of Xplayer to render the video previews as icons. The poblem here is that Xplayer works only with graphics acceleration, so it crashes.
Well... I found a workaround for this bug on my laptop (that have that ... "Intel Corporation 3rd Gen Core processor Graphics Controller" or i915) just by uninstalling Xplayer.

I found that Xplayer was the cause of the bug when I disabled Nemo previews on its preferences, then opened the folders normally but without previews.
So, I opened a video by VLC Media Player normally.
Then I tried to open the same video with Xplayer Media Player and... well... it crashed and I just needed to log in again.

So here is the point: There should be a bug regarding Xplayer (or its dependencies) that is causing this instant logoff on the first version of Linux Mint 19.
Uninstalling it, you will be unable to have video previews on Nemo, but at least, it's a workaround for this annoying bug (at least until the bugs regarding Intel Corporation 3rd Gen Core processor Graphics Controller are fixed.)

This is the Linux Mint Cinnamon version that I was using when I did this workaround:

Kernel         : Linux 4.15.0-20-generic (x86_64)
Version        : #21-Ubuntu SMP Tue Apr 24 06:16:15 UTC 2018
C Library      : GNU C Library / (Ubuntu GLIBC 2.27-3ubuntu1) 2.27
Distribution   : Linux Mint 19 Tara