Thank you. Your support and the hardware repair service confirmed that it's no hardware defect, just a problem related to the impedance of the headphones. I will try it with other ones. So everything is fine. I used Sennheiser HD-600 by the way.
I bought a Virus Indigo2 and now realized that there is noise at the PHONES Output. The noise level is not influenced by the Master Volume Knob. It always remains the same. When I switch sounds sometimes there is a crackle, too. The noise sometimes even stops after that for half a second and then kicks immediately back in.
On the other outputs there is no noise.
Do you have a clue what the problem is and how I could fix it? Thanks in advance!