I get these alarms quite often on older, R-J3 controls. Most of the time they are easily resettable just by pushing and releasing the pendant ESTOP button and issuing a reset, but sometimes we have to go into the system configuration menu (line 29, I think) to get it to clear.
Basically, the dual chains of the ESTOP circuitry are not opening and/or closing within the expected timeframe. It can (and in our case, does) happen for no apparent reason. I have even been able to generate this alarm by turning the AUTO/T1/T2 key switch too slowly or feathering the dead man switch while in teach mode. You could try rewiring your ESTOP circuitry and replacing buttons or switches, but my experience is that some controllers are just a little more susceptible to chain failure than others for whatever reason. There are 2 or 3 different ways to reset a chain fault, and I would recommend teaching them all to whoever is responsible for keeping the robot running.