Well, I manually created the shortcut and now everything seems to work as it should. I am able to import the waypoints and then import the flight plan, without having to manually copy the file every time. Thanks for the hint!
These 2 things aren't what is needed to successfully import the flight plans? Why should I enable the other export item that points to the nonvol DB? It even points to a non-existing file name (sys_nand0), when the existing ones are sys_nandi and sys_nandj in the folder "H:\FSX\Garmin\TrainerData\GTN\nonvol"