Thanks anyway !
OK. Now I bought a T5 screwdriver and opened the player (very carefully, of course).
Unfortunately I found the daughterboard and motherboard being soldered on 4 places (Two times each, just "above" the two screws holding them together). So I couldn't separate the boards to check the soldering.
I didn't know what to do next.
Shall I open these four solderings, check the called areas (J1 - J4) and then start the resoldering? (Do these solderings just keep the two boards together, or do they also conduct energy?).
Is there a way of preventing tin-solder from running across the tiny connections on the boards?
I asked some electricians but they all only told me to send my player back to Iriver Europe. So I guess I'll have to do it myself.