OpenSCAD suggests that "object may not be a valid 2-manifold" but it looks perfectly fine in OpenSCAD and in Meshlab.
Have looked through the file several times, and cannot find any point or line intersections, they all overlap in 3D.
Is there some useful software for diagnosing this? It seems to me it ought to be trivial to for example highlight the offending volumes, rather than just print a general error message with no details whatsoever.
Edit: slicing it in Cura works fine also.