Haven't looked into the cause at all, so not sure if it's a bug, an edge case, or just something weird in the particular setup, but in this screenshot posted on Discord by GigaLem, the rank graphic has a "Rank graphic not found" image instead of generating a placeholder.
(https://media.discordapp.net/attachments/598671586203271219/1037654236772700211/image.png?width=1119&height=629)
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.