Arduino relay module interferece on signal pin (230VAC moror load)
There was a similar problem with the relay data.
In my case, a separate power supply to the relay unit helped (with common ground).
Most likely, the interference goes precisely along the power line, where the relay coil and the arduino power supply are combined.
Also, when switching an inductive load between the relay contacts, sparks can occur, which can also sometimes freeze the arduino. For these cases, a EMI/RFI supression capacitors can be installed parallel to the load. (Idea taken from the internet).