Found the cause of this, and fixed in commit ff3a702.
If you are experiencing this bug, go into your NL installation's "gfx/menu/" subfolder and delete any file named "rank_graphic.png" or "group_graphic.png". This file may exist from older versions of NL; and the way NL handles loading menu screen graphics is "attempt to load graphic file from pack, if this fails, fall back to gfx/menu/ directory". It was only if this too failed, it would fall back to auto-generating it (which on a clean install, this would fail). I've now made some changes so that these two files would only be loaded from the pack, not from the default graphics.