Ok,
so I found the solution: I had installed the dummy-dev package, which (for the Cxxx) at least creates a dummy network so that NetFront does not ask you if you want to start a network connection. Looking at the package content, it installs a kernel module (which does not work, since the C3100 uses a 2.4.20 kernel) and somehow interferes with the IR interface. Once I deinstalled it, everything started working again.