Package inputenc Error: Keyboard character used is undefined - ^^c0

The class file diploma.cls is not well written.

Packages should generally be loaded after \LoadClass. You can just move the \LoadClass declaration at the top. Also pdfx should be loaded before inputenc.

\def\fileversion{9.0}
\def\filedate{2016/06/20}
\def\docdate{20. června 2016}
\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{diploma}[\filedate\space \fileversion\space diploma EXPERIMENTAL LaTeX document class]
\LoadClass[11pt]{article}
\RequirePackage[a-1b]{pdfx}
\RequirePackage[latin1,utf8]{inputenc}

I don't think the latin1 option to inputenc does anything good. Actually, the package should be loaded in the document and not by the class.