For a while now, my installation (on a Thinkpad P51) has been doing weird stuff. I think it might have started after I updated to 18.04 from 17.10, but I also vaguely recall having encountered a similar problem (could have been the same) before the update.
The problem: While logging out from 'normal' ubuntu, and trying to login to i3wm (and the other way around, too), when I enter my password, it always gets rejected the first time, and seems to always be accepted on the second try. After confirming the second try, I am faced with a purple screen and a cursor that still moves. Multimedia keys still show their graphical effect. That is, the usual gray popup/overlay on normal graphical layout, not in i3wm because there multimediakeys keys are differently configured. Something else worth mentioning is that, when the purple screen is displaying, ctrl+alt+F1 takes me to the login screen again, sometimes with the option of choosing the UI/layout/window manager, sometimes without. This keycombination seems to do the same for normal interface as well as i3wm. To me, the login screen that is then shown sometimes seems like the screen after locking, and sometimes seems like the screen after logging out.
I have looked at multiple other threads (such as freeze in purple screen after login. and Ubuntu 18.04 stuck at purple screen after login which seemed somewhat similar), but did not find any description that matched what I mentioned above.