Bernd DF9RB from the DL0AO team proposed extending the display range of the wideband grabbers to 48 kHz:
 http://df6nm.bplaced.net/dl0ao/VLFgrabber/vlfgrabber_dl0ao_test.htm

Thereafter, we stumbled upon four unidentified strips on 38.0, 40.4, 42.5, 44.2 kHz. My first thought was that these must be local interference. But it turned out that they show pronounced nighttime fading, and they are also visible on Peter's grabber in Kiel:
 http://lf-radio.de/cgi-bin/ki/show_wf.cgi?date=17-05-26

All four frequencies carry the same 200 Hz wide spectrum. Zooming in exhibits a fine structure consisting of a number of closely spaced discrete lines, suggesting modulation with a constantly repeating bit pattern (see second grabber window at DL0AO). There are occasional bursts where the signal goes up by 10 dB for several ten minutes, like yesterday around 21 UT. The red or purple colour-DF would indicate arrival from East or Northeast.

Any ideas what these may be?

73, Markus (DF6NM)