try ...give the specific files name. no wildcard *.shp.

Maybe it works.

Thanks for the reply f.kemminje. I did try that before I asked. There's probably some random little thing that lastools doesn't like. Just not sure what

Have you tried opening the shapefile in another application? Any chance you can share it here?

Can the shapefile be read by ArcGIS? If so then lastools requires a non-standard shapefile. There are some exceptions - see https://docs.safe.com/fme/html/FME_Desktop_Documentation/FME_ReadersWriters/shapefile/troubleshooting.htm

Update, I can get it to work if I use a geometry validator before I write it. Use all settings (but remove the two OCG checks), turn on checking for self intersecting, and I also remove vertexes closer than 0.1m (but I don't think you need this last step). I also dissolved before the validator in one case

Second update, I had this problem again for a different reason. This time it was fixed by having the Geometry in the shapefile write set to the appropriate type. Shape_polyline for my new case but probably shape_polygon for the case of this question