OK, so it all seems to indicate that there is capacitive crosstalk from the left channel to the input of the right channel.
Possible receiving parts: R13, C2, R6, R3, C4, right-channel positive op-amp input, all wiring and copper islands and the connector that are connected to either side of C2.
Possible transmitting parts: anything connected to a left-channel node with a signal on it, probably an amplified signal:
Left-channel op-amp output, R40, R70, R80, R90, C60, C50 (side that is not connected to the negative op-amp input), R110, and their connecting wires, connector and copper islands.
On the photograph, the right channel input cable gets close to R70, but only to the side of R70 that is connected to the negative input of the left-channel op-amp, which carries a non-amplified signal. The dampers are soldered straight to the input connectors and the right-channel input connector is adjacent to the left-channel output connector. Does cutting out R3 and C4 or putting a shield between the left output and right input connector help?
If you should have a couple of capacitive crosstalk paths that are about equally strong, eliminating one with a shield will only give a minor improvement. For example, suppose there is capacitive coupling from the wires of R70, R80, R90 to C2 and about the same amount from the left output connector and its resistor to the right input connector and its damper. You then have to shield both to get a substantial improvement.
Possible receiving parts: R13, C2, R6, R3, C4, right-channel positive op-amp input, all wiring and copper islands and the connector that are connected to either side of C2.
Possible transmitting parts: anything connected to a left-channel node with a signal on it, probably an amplified signal:
Left-channel op-amp output, R40, R70, R80, R90, C60, C50 (side that is not connected to the negative op-amp input), R110, and their connecting wires, connector and copper islands.
On the photograph, the right channel input cable gets close to R70, but only to the side of R70 that is connected to the negative input of the left-channel op-amp, which carries a non-amplified signal. The dampers are soldered straight to the input connectors and the right-channel input connector is adjacent to the left-channel output connector. Does cutting out R3 and C4 or putting a shield between the left output and right input connector help?
If you should have a couple of capacitive crosstalk paths that are about equally strong, eliminating one with a shield will only give a minor improvement. For example, suppose there is capacitive coupling from the wires of R70, R80, R90 to C2 and about the same amount from the left output connector and its resistor to the right input connector and its damper. You then have to shield both to get a substantial improvement.