Just to close off this thread:
It was indeed the roll lock lever sensor, when that's gone, it can't tell if the lever is down, and won't proceed with loading the roll. So, a $2.70 sensor stopped everything.
It's a p.i.t.a. to get to, but a good engineer can do it in short order. Just make sure they bring one of the sensors with them, if this sounds like your call out issue.
(To be clear: it was the legs of the sensor that are very fragile and break away, so it's no longer in the right place, most likely from the jostling from the roll lock lever.)
Hope someone finds this helpful in the future.