Try creating a new project:
File > Project > New.
And press "Use Defaults" to create the project folders.
Go to UV snapshot options and check that it's using the new project folder to save the file to.
Sometimes when Maya doesn't create image files, it might be becasue it needs a proper path. There may be someting wrong with the path you're currently using (even is it's the default one).
If that doens't work:
Try changing the format of the image
or
Try creating a new material and applying it to the mesh you want to take the UV snap shot from. (This works when Mental Ray fails to bake a texture). The error usually occurs on imported meshes.
other than that, I'm not sure what else to recomend.