Ok, I unlinked the audio library and installed the plugin to my internal drive,

after changing the ownership to root:admin the plugin was found by logic.

My external drive has a ownership "username":staff. Maybe it's an security problem ?

I would guess it is a permissions issue, possibly related to sandboxing (security protocol on macOS). It is possible that Logic X will refuse to load plugins that are not in specific locations (e.g. the default AU location on macOS), or which do not have appropriate permissions such as "username":staff. Changing the ownership of the drive could fix this, but be careful (though it seems you have some knowledge of permissions). It's also possible that the drive you have is not in APFS format, and it needs to be (this is purely speculation on my end).

-Tom