I have absolutely no idea what happened.
There was nothing in the GThumb preferences about not showing thumbnails, so I was a bit at a loss. Fortunately I got the idea of using the Gnome Configuration Editor, which on my Debian testing system is on Applications | System Tools | Configuration Editor and there it was. The gconf key /apps/gthumb/browser/show_thumbnails was not set.
Once the key was set to ‘true’ (by checking the box) GThumb immediately showed the thumbnails again.
So the fix was easy, but I still haven’t got a clue why it was turned off automatically in the middle of a session. (more…)