As I assumed, the stl triangles are really strange... Netgen gives a warning
WARNING: File has normal vectors which differ extremly from geometry->correct with stldoctor!!!
If you click on Geometry -> stl doctor -> show stl-triangles you can see what strange stl mesh blender produces
My guess is that Netgen cant handle that thin triangles.
You could try to create "nicer" ones using some existing library but I think it shouldn't be too complicated to create your own little svg to stl converter which gives better stls...
Still it would be interesting to examine the error and maybe catch it. On my PC Netgen is unable to mesh one of the surfaces, it refines a lot to the edges but is not able to build the surface. My guess would be that the edge detection somehow fails because of the strange triangles and normal vectors. Because of the user meeting I do not have much time this and next week, but I'll look at it afterwards
If you want to have a look at it: the main code for meshing stl geometries is in netgen/libsrc/stlgeom/meshstlsurface.*pp - this generates the surface mesh from a stl geometry. If the surface mesh is done a volume mesh is generated from the surface mesh with the standard mesher.