I believe the error was in the safety relay, not the PLC. Maybe your point was that an error on the PLC program is what was triggering the error in the safety relay, and that may be something worth looking into. As far as I knew, the SLCs never had a qualified Safety PLC format, so it's entirely possible that there could be something that doesn't happen correctly inside the PLC that is then triggering the safety relay, then in the process of cycling power every time to clear it, the safety relay eventually gets its own internal failure and has to be replaced.