I’ve found how to solve it !… by doing nothing !!
Explanations : The problem of red tracks occurs when transferring with sync manager to an external drive. When transfer is complete some tracks are red on the engine desktop view of that drive (which seems related to special characters). Trying to load one of them from the drive into the built in player doesn’t work.
BUT if I unmount the drive after the transfer, and mount it again, all tracks are still there on the drive view, and none of them are red again 
Using the drive in the Prime 4+ is ok, all previous red tracks work perfectly.
It seems that it is a « first time issue »… which has lost me 2 days of searching 
So definitely a bug that needs to be fixed, but hopefully everything’s ok now.
For the record, this happens whether the track has been imported from Traktor library, or directly from my hard drive.