[DiapDealer]

I can't see anything wrong (epub spec-wise) with your process using the latest version of Sigil. For epub2, the only requirements for a properly configured cover page is a manifested xhtml file which has reference entry of type "cover" in the guide section of the opf file; and a properly manifested image file that has a meta entry with a "name" attribute value of "cover" and a "content" attribute value which matches the manifest id of the image. All of which seems to happen with the steps you listed.

The only thing I can think of is that Nook has special (non-spec) requirements that calibre automatically accommodates in an epub to epub conversion. Or there's something about the Add Cover tool's svg wrapper that Nook doesn't like.

If you're talking epub3, you may have to manually run the Update Manifest Properties tool (Tools->Epub3 Tools) to make sure the svg manifest property gets added to the cover page's manifest entry in the OPF (though that may automatically get taken care of with the Add Cover tools--I forget).