OpenSCAD "not valid 2-manifold" useful information 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.
Occasionally, a model created in OpenSCAD will have, as you suggest, overlapping segments. Often enough, this will generate the message you receive. OpenSCAD has an implied union function when it comes to independent objects, but if you wish to clear those errors, experiment with explicit union statements where you have joining objects. It may be necessary to join only one pair at a time to clear the error.