Thunar was updated today (2018-10-08) from 1.6.15 to 1.8.1 and while the screenshot issue disappeared (https://tracker.pureos.net/T531), now it crashes immediately when clicking on any file. Crashing does not happen if logged in as GNOME on Xorg.
I installed Thunar when GNOME Files (Nautilus) caused a total system freeze. Nautilus got updated and is stable for now.