You already posted on pixls.us where the Darktable developer hangs out. The problem has been reported before, all over the place. see this one: https://discuss.pixls.us/t/partially-sol...r/46609/11 It looks like a Windows issue.
You could try going back to an earlier version of Darktable (if you can find one that works) Probably the best bet is, get your image open in Darktable, do whatever editing you need, export that as a high bit Gimp .xcf (or a high bit tiff ) and open that with Gimp.
You could try going back to an earlier version of Darktable (if you can find one that works) Probably the best bet is, get your image open in Darktable, do whatever editing you need, export that as a high bit Gimp .xcf (or a high bit tiff ) and open that with Gimp.