Common sense is what I use when I go trailriding in here in Sweden. I regularly use logging roads and trails made by forest harvesting machines. I can defend this any day, since the logging company already has done the damage and not me. The only thing I worry about is people picking berries and mushrooms. No joke. They show up out of nowhere. Beware of riding during hunting season as well. You won´t likely be mistaken for a moose, but hunters get really pissed off when you stir up the game... These activities take place in late summer/fall, so they can be avoided. Bring a cell phone and know your position. Coverage is excellent in most places, should you get yourself into trouble...
Of course, you could always contact one of our numerous MX/Enduro clubs for some completely legal riding on one of their approved enduro trails.
To sum this up, don´t sweat the details. If you want to ride in some remote part of our country then just go ahead and enjoy it. That´s what I do. I can´t speak for the rules and customs in Norway though. But I sure would like to ride there some day. Check out the Hardangervidda highlands!