"CI2" to "Fender Mustang III" connection produces noise/hum

Dear reader,

I got my new “Mustang III”, but when I connect the CI2 via the aux-connector, I get pretty much noise/hum.
So, I cannot use this setup of equipment at all. Do you have any ideas?

May be, I have to add an important topic: If I connect CI2 to my “low-cost Roland-Micro-Cube”-Amplifier, then I have no such problems. I am puzzled