I also had the same issue after an upgrade to the latest version. It cost me hours until I recognized, that the Modbus Salave only is in the 'Running' state if an active connection is established to the slave! If no Modbus Master is connected to the slave, the slave will remain in the 'Not Running' state. Also check the 'Status Page' of the Modbus Slave and check the Connections, Request and Exception counter.