I've made the test on my wife's computer, allways the same problem with my phone... but not with my wife's or my son's phone !
so the issue comes from my own phone !
I have re-initialized it, delicate and long process, and now it's ok !!
something went wrong, something vicious (it worked with windows explorer, but not with MB), I don't know exactly when, don't know why, but now it works ;
really sorry for having annoying you with a local problem, I hope it can be helpfull for someone in the same case ...
I profit of this to tell thank you Steven, for this excellent soft that is MusicBee !! (why a Bee ??)
wish you all an excellent week-end