sound effects are not shown anymore in Final Cut Pro X

This problem occurred for several users with FCPX 10.3 but no real solution was found. I got all sound effects in the proper folders /Library/Audio/Apple Loops/Apple/Final Cut Pro Sound Effects but starting FCPX 10.3.4 does not exhibit any content.

This is on a Desktop Mac with OSX 10.11.6. I might be correlated with El Capitan as with my MacBook with HighSierra the same FCPX version runs fine.

As I am not inclined for a "solution" to update the OS I wonder whether anybody else found a solution in EL Capitan?

Thanks, Helge