I tested it again and it seems like the invalid resluts appears more often the less area is coverd with navmesh. With a small navVolume and the count of invalid results increased so much that valid resluts became the exception.
But even with a navmesh covering the whole place it gets invalid results something about every 40 calls.
It might be trying to set a point on the walls and can’t cast on any navArea?
I use the starter content map (I’m aware of the navArea not covering the whole place in the screenshot), the transformation handles is where my simple test actor lies.
there are 2 custom navigation areas, 2 NavLinks, a custom pawn and a standard 3rd person pawn.
[Output Log][1]
THX for your help, I attached a the outputLog as a txt 
I let fire a custom event every second to start the print that give these values: