So it may not be a relay at all.
BTW the relays will look like this..
https://www.digikey.com/en/products/...C2-5NU/4291118 (likely the actual part number).
But a quick test if you are inside it already is swap the channel cables to/from the input board and see if the problem moves to another channel. IIRC output channels may be pairs on a connector input channels singular. But you can try combinations that make sense.
Electrolytics can die without bulging or leaking, the real way to tell is with a capacitance and ESR (equivalent resistance) meter, often disconnecting the cap first. Or by looking at signal or power supply that is not being filtered properly because the cap is not working... best with a oscilloscope but sometimes you can use a multimeter to measure the AC voltage ripple vs DC voltages esp. on a power supply rail filtered by these caps. All these 003 are of age where electrolytic caps failing becomes an issue, those damn things cause so many problems in so many electronics as they age.
These boxes also have so many connections, and they can suffer from corrosion or loose connection pins, which is what Janne was after you reseating stuff and sounds like you tried.