2016-02-10 And now vectored VDSL doesn't mind transmitting
After a restart of the modem the interference starting around 16.5 MHz hasn't returned. The interesting fact is that now transmitting on 14.070 MHz PSK31 does show in the signal/noise graph of the modem but there was no connection hickup the first time I did that, the bits/carrier just adjusted down and things went on fine.
Vectored VDSL spectrum as reported by the Fritz!Box 7360. The earlier interference seen starting around 16.5 MHz isn't visible.