ofn-erase-background

[Ofnuts]

Your final image is normal as an intermediate step... but it is what I get when removing the color-to-alpha step from the script. There are only three lines of code at play there:

Code:

pdb.gimp_image_select_item(image, CHANNEL_OP_REPLACE, savedSelection)
pdb.gimp_selection_grow(image,2)
pdb.plug_in_colortoalpha(image,layer,(r,g,b))

The first one is the verbatim repeat of another operation, so I don't see it failing the 2nd time around, the second is too trivial to be a suspect, which leaves us with the third one. Have you got a functional Color-to-alpha plugin?
 
Also AFAIK if the code crashes you'll get a message. If you have the error console the messages are quietly logged there and easily overlooked.

Have you got a saved selection channel?