"Signal lost returning to launch location". Could be a physical problem with the drone. I recommend contacting Gopro Karma support. They can look at flight logs and analyze the issue. If the Karma is still under warranty they can replace parts as needed.
Make sure you have a good GPS signal. Also be aware a concrete surface contains a steel rebar, which will cause problems. When you take off, let it hover for awhile, if it remains stable, this is a good sign of a good GPS. If it wanders bring it down and relocate launch site.