Running UR242 only with ipad - hum noise

I try to start using a UR242 as an audio interface for my ipad. There is a lot of hum noise compared to using the same interface with the same guitar at an Apple iMac. Even the level meters of the software dspmixfx or Cubasis show this noise. As soon as I touch the housing of the interface the level of the noise gets much lower. The interface already was replaced. So I start believing that this is an design error of the grounding of this interface. I already tried a lot … another ipad, di-box, putting an usb-hub between the ipad and the ur242, changing cables. The only part of my setup that I didn‘t change was the Apple usb-lightning adapter (camera connector …). Any other idea? Running the interface at my iMac it sounds great but I already have an interface for this computer.

… and by the way… of course it still has to be powered by the power supply… the interface is not bus powered.