Wiki

As DVI and PS formats are not that common any more, we decided to drop support for plain latex and stopped using a package that could change letterspacing (soulutf-8) for that engine.

Feel free to use any older version of classicthesis.sty that works for you. As an alternative, you could just put back the old code, say after classicthesis.sty has been loaded:

#!latex
        \ifthenelse{\boolean{pdf}\OR\boolean{xetex}\OR\boolean{luatex}}% ever used? only dvi engines remain
          {\relax}{%
            \RequirePackage{soulutf8} % for letterspacing
                \sodef\allcapsspacing{\upshape}{0.15em}{0.65em}{0.6em}%
                \sodef\lowsmallcapsspacing{\scshape}{0.075em}{0.5em}{0.6em}%
                \DeclareRobustCommand{\spacedallcaps}[1]{\MakeTextUppercase{\allcapsspacing{#1}}}%
                \DeclareRobustCommand{\spacedlowsmallcaps}[1]{\MakeTextLowercase{\lowsmallcapsspacing{#1}}}%\protect
          }%