Not sure if this is specifically a 5.1 issue, however I thought I may go through and remove any add-ons I no longer use. I removed the following add-on (thinking it was strange, given I never installed it):
Turns out this was actually the Modbus binding, and everything modbus stopped working!! Reinstalling it, got all my Modbus devices working OK again. So it just looks like the title in the add-ons is incorrect, but works just fine….
I am making a big assumption that this appeared in 5.1, given that was where this turned up:
