ePub to Mobi Conversion in Calibre Problem

I've been using Calibre for years and never encountered any real conversion problems. However, lately, I've tried to convert some ePub books to Mobi to put them on my Kindle PaperWhite and, while they seem to convert just fine as far as Calibre is concerned when I view the Mobi file either in Calibre or on my Kindle all I see are rows and rows of dots.

I also tried to convert the ePub to just text or just PDF with the same result. Is there something special about the ePub file that would break Calibre for all conversions?

Thanks!