Bug#406971: found kind of solution
Hey. I've made a new installation, but this time in expert mode.
I kept it all the same, just installed the kernel 2.6.18-3-486 instead
Now it's working fine. So I think it must have something to do with the
linux-image-2.6.18-3-686, and not with the debian-installer.
I installed the 686-kernel afterwards, rebooted with the 686-kernel, and
now it's running.
No idea what happened there, the hardware stayed the same, the
installation was the same, just the kernel was different.
Hope that helps,