
I tried a much larger HDR image instead of the .png - and not only did it work; the .png suddenly worked too when I switched back!
Does this make any kind of sense?
EDIT: I repeated the procedure (switching between the HDR and .png), but now it didn't work...

EDIT2: And now it works again. In other words, the whole thing seems rather unstable: One time out of three, I get the correct render; the rest of the time Octane either renders the cube without the image OR it doesn't render anything at all...