Guys,
my Kemper is not defective. It was... a cable. I´m embarassed. When I checked that part of the equation with spare cables, I was on a sound which turns out always sounds bad. Wiped the profiler before, left my sounds on the computer. Thought that sound would really show the problem.
The bad cable was touch sensitive, and did not show the problem when I hooked it up to another head.
Kemper support was really helpful and had a calm systematic approach, which I was lacking.
Keep your calm when doing studio housekeeping chores.