That "EXT(RGB)" seems a good yes , indeed! :-)
I have here a LG 19LS4R that doesn't works with the STB in RGB mode (looking now at the monitor/tv schematics)
Edit- just for the record ... after looking at the schematics, even my LG supports RGB over SCART.
Everything pointed in a defective SCART cable (red channel wire knackered) :-)