finally! i managed to get a log:
/etc/event.d/bluetoothd: exec /usr/sbin/bluetoothd -n -d '*'
what i did: autoconnect to headset and then i tried to listen to music, music was played via headset, but in very poor quality (wrong bt profile? display from headset said i was phoning).
i will try to get a log from the usual disconnect when telephoning, but i may need a while (week?).