It is on a Win10 box but with TeXlive installed and not MikTeX.  Preview
uses in this case `rungs.exe' (TeXlive's counterpart of mgs).  I've set
the variable `preview-gs-command' to gswin64c.exe and indeed it didn't
work as well.  My current guess is that running `pdf2dsc ...' somehow
fails.  TeXlive ships a small executable called pdf2dsc.exe which seems
to do the job.  Does MikTeX comes with such a thing?

Best, Arash