Try swapping the ULN2803 chip from a different set of channels. If the problem goes with the swapped chip, the problem is likely the 2803 chip itself. If that doesn't work, try swapping the 595 chip with a different one to see if the problem moves with the 595 chip.
If you're referring to the SSR channel itself (instead of the LED on the 595 board) and the above has no effect, then the issue is likely the SSR -- the Triac may be bad for that channel, or the MOC chip, or both.
In any event, I think these actions will help you identify where the problem is and you can replace that part.
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