Issue closed,
Adam explained to me that the "lock" led is flashing red when you turn on the BR-20 and the internal computer is booting up and it turns solid once the sequence is complete. In my case it was flashing also during the playback probably because I was starting playback too early or maybe because I was frequently adding files to my USB Disk.
Now the led is solid green during playback, as it should be.
Thanks again James.