You have a problem with some electrical noise in the system, induced by the Relay-board.
How does the underside of the relay-board look ? I have seen some relay-boards with insufficient insulation from the low-voltage to high-voltage side, making for dangerous situations and problems all together.
For example, look at this board - the one of the high-voltage pins is 2-3mm from the pin header to arduino - dangerously close!
// Per.