It seems to me that it is a problem of using the stringtest/0da228...81c rather than the a Path() object. This means that the separator / is incorrect (on Windows). So I suggest trying to change name column to contain paths.
I have a similar problem - I’m using an absolute path, and fastai is adding on .\ before it.
So I get paths .\\C:\\dir1\\dir2\\image.jpg which throw an error.