Benno_fra_dk:
Please take a look at this video, and watch the signal lights on the arduino:
Dropbox - Error - Simplify your lifeI am powering the arduino from a different outlet (7,5v) - I think its even on a different phase in the house. It shouldn't do that, should it?
I would suspect your relay boards. Can you remove one and photograph the board in detail - good sharp pictures of the top and underside of the board.
My best guess is your boards are manufactures wrong, and offer no real isolation.
// Per.