c1iscex has not been running for a couple of days (since the power shutdown at least). I was assuming that the problem was recurrence of the c1iscex IO chassis issue from a couple weeks ago (5854). However, upon investigation I found that the timing signals were all fine. Instead, the IOP was reporting that it was finding now ADC, even though there is one in the chassis.
Since I had a spare ADC that was going to be used for the CyMAC, I decided to try swapping it out to see if that helped. Sure enough, the system came up fine with the new ADC. The IOP (c1x01) and c1scx are now both running fine.
I assume the issue before might have been caused by a failing and flaky ADC, which has now failed. We'll need to get a new ADC for me to give back to the CyMAC.