Everytime I have used my MP3 player- or digital camera's USB cable,
and am finished with it, I get kicked offline.
I then have to unplug the modem cable, and plug it in again.
Only then I can get back online.
Does anyone know how this is possible,
and maybe what I can do to fix this?
When you unplug the mp3 player or camera, do you unmount the device first?
If you're using Windows, then look for a small green icon on your system tray (the part next to the clock) when your camera or mp3 player are plugged in. Right click on it and choose unplug/eject hardware, select the device you want to unplug from the list and hit "Stop" (or "Unplug/Eject", I forget which). Then click "OK" (or whatever it says) and remove your device when Window's tells you that it's 'safe'.
I don't know if that will help, but it's worth a try.
Edit: I'm presuming that your modem is also attached to the USB bus.