Latest status update. Unfortunately the problem is back. I am now 100% sure it is the Kemper itself that causes the problem. Guitar in at the from input, headphone out at the front and I still hear the all distorted sound. There are no XLR cables or other components involved.
I am now arguing with the shop where I bought the Kemper that I want to replace it. The are reluctant to send me a replacement, which I think is ridiculous as I bought it on Februari 25th. I did make a small movie of the problem occurring and will try to post that later today to this mail. To be continued.
Oh yes, I have also logged a ticket with Kemper a week ago. I send them the movie showing the problem. My first priority is to get a replacement before the gig I have coming Saturday...
Hope you get it resolved soon dude. Keep us updated, how it went with your retailer and Kemper support. Good luck.