the RGB cable in an Amiga so called "genlock" is a tricky component. If i've understood you correctly, the HAMA have a not shielded flat cable (that could be damaged due the age)
Try with a multimeter a continuity check test between the connector and the place where it is soldered (some of these cables carries a IC inside the connector, to compensate/trigger the external clock signal which is vital for a correct functioning)
good luck!
just for ... the EXTCLOCK (/XCLK) and CLOCK Enable (/XCLKEN) are the pins 1 and 2 of the RGB Amiga port/connector
RGB are the pins: 3-4-5
Composyte SYNC: pin 10