One of the problems is that the piston system locks up if the gun is taken outdoors from a warm building when the humidity in the air freezes in the Norwegian arctic conditions. The C8 (Colt Canada full-auto AR-15) that is used by the Norwegian special forces also has this problem but can be fixed in a much more timely manner than the HK416.
Apparently the gas regulator, which that controls the amount of gas flowing into the gas piston system, often switches modes during firing. The gun will not function properly if set to the suppressor mode if a suppressor is not being used.
I imagine the Norwegian will have these problems sorted out in the next few years. It does seem odd that they did not identify the problems before purchasing the rifles.
Many thanks to Jay for the link.