I double clicked an image, Thunar started to consume a lot of processor power, and just before I thought it was frozen for good, it said "Couldn't open Mirage". I had temporarily installed Mirage, and later removed it again. Thunar should never try to use non-existing applications.
Maybe an old desktop file was still present in the .local/share/applications folder. Tried that and it directly tells it cannot execute the process, so that's fine.