Beckman LS6500 Rack Motion Error

I posted this back in 2018 but the responses seem to have disappeared and I am having the same problem again. What is the most likely problem of a Beckman Model LS6500 rack motion error message? The racks move on the belts, but the chain does not move. I see some replacement circuit boards available on eBay such as Chain Dog Sensor and was wondering if replacing that board may be the answer. Any help is appreciated!