The M1 supervises the data bus and polls for the bus devices multiple times per second. After several failed polls attempts if the device does not respond the M1 will Log A Missing Expansion Module. In the logs which device went missing? The M1DBH Data Bus Hub is a passive device with no circuitry simply keeps Data A and Data B connected in a Daisy Chain configuration. I don’t believe the M1DBH has gone bad but you could have a loose or intermittent connection, bad butt splice or bad RF45 connector. I could also be possible the M1 took a slight surge that interrupted the communication over the bus.