finally! i managed to get a log:

what happens from usersight:
n900 is not freshly restarted, it was before the log connected to the headset, but disconnected by the phone (dunno why it did that).

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).

restart headset AND n900

autoconnect, went wrong in the usual way: first shown as connected, tried to play music, music was played via internal speaker, headset disconnected

i will try to get a log from the usual disconnect when telephoning, but i may need a while (week?).

and i want to say a big "THANKS" to luf because you are trying to help me and figure out what the problem is.