[repilo]

This works for me:

1. Open the ePub with Sigil and extract the cover jpg.
2. Open the jpg with Gimp image editor and then:
   File->Export As..., “JPEG Image”, [Export].
3. Now in the “Export Image as JPEG” tab:
4. Adjust cursor “Quality” to reduce the output file size.
5. Uncheck all options except “Save color prpofile”, “Optimize” and “Progressive” (under Advanced Options), especially uncheck the option:
   [ ] Use quality settings from original image
   
6. [Export]
7. In Sigil, reload the jpg back into the ePub.
8. If it still does not work, in Sigil open content.opf, look for the tag
   <dc:identifier id=“BookId” etc...
   and delete it.