Kudos to Tom for finding the explanation
Apparently the PC-4 Evo draws current in the first half of the sinusoid and returns the unused energy in the second half of the sinusoid. So the current is ahead of the voltage by 90 degrees just as Tom said. Although the total consumed power is very low (no consumers active) the standard measurement equipment which electricians use shows the drawn energy without the returned energy.
Now I know it is not my power conditioner that eats my energy.
Let me also mention the help from Gigawatt. I sent them a mail with the problem and they replicated my scenario in their lab, sending me all the measurements and explained why I get the wrong readings measuring power consumption. Great service!
Best regards
Ecki