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The most likely issue on the index is that the encoder differential
receiver (U6 ISL32273) or possibly U13 (74ACT04) have been damaged

On the other 7I76, do you mean 1 isolated input does not work?
This is odd because there is a relatively high power 10K resistor
in series with each isolated input so even a short connection to
120 VAC  is survive-able. It might be a contamination issue.
Perhaps the resistive divider and filter cap for the specific input
could be looked at closely