Saving images as PNG files fails without any error message

When using the ‘Save As’ option to save an image as a PNG, the save operation appears to successfully complete without any errors. However, no PNG file is saved to the specified folder on disk.  The only other meaningful observation is that we don’t see an interim save PNG dialog after pressing the ‘Save’ button as we might with a JPEG file.  Saving as a JPEG file works fine, but there is also an interim dialog asking for preferences regarding quality and so forth.

We need to save images as PNG files to save the transparent backgrounds.  Any suggestions as to why we cannot save the images in PNG file format?