The Nyquist-Theorm behind the fft
"Why do I get here a mirroring at the 400 Hz? Does it something to do with the symmetries of the fft?"

yes.  the FFT is a "fast" method of computing the DFT.  the latter half of the DFT corresponds to what we normally call "negative frequencies" and, for a real signal, the spectrum in the negative frequencies are a complex-conjugate mirror image of the positive frequencies.