I also figured that the Arduino was getting power from the USB. Just like you said. So I checked once more whether all connections were good. And apparently the GND wire of the Arduino was not connected, at all!!
You wouldn’t believe how silly I feel at this moment!! But on the other hand I am so incredible relieved!!
Now everything works like a charm. I am now only waiting for the camera trigger cable, and I should be able to start shooting.
And in the meanwhile I can install everything neatly in a project box which arrived last week. And assemble a rig.
Thanks a million for sharing your project and for being so kind and patience to give me some help when I was stuck! It’s really appreciated.