I bought the unbalanced cable ( https://www.audiophonics.fr/fr/cabl...-femelle-5-broches-2-rca-noir-15m-p-4721.html) because the unbalanced connection is working on my turntable and the preamp.OK. From the diagram and photos above, the tonearm cable seems OK. I'm thinking the problem is the phono preamp.
If the phono preamp is really balanced and if it also has RCA inputs that work, and also if the balanced input is properly designed, then if one wire were disconnected from either pin 2 or 3 of the XLR, in balanced mode the phono section would not play on that channel. So I think this is worth checking.
I still have the "bad/defective" balanced cable that I am planning to fix. I have no idea when I will fix it. I also have to solder 4 Grado headphones. The Grado job has priority.