The current version of Skimp cannot import PLY files unless they contain polygon face and mesh data. Some PLY files are only point clouds and do not have face data. These types of files will currently cause a bug splat or crash when you attempt to import them.
We plan to update Skimp in the future to display an error message rather than crashing this way.
To resolve this issue, first convert your point cloud PLY file to a mesh using your 3D scanning software. Then, Skimp should be able to import it.