Stylesheet does not capitalize Polish letters correctly

the issue is in the class file, which uses \uppercase for the case changing rather than \MakeUppercase. A fix:

\makeatletter
\def\@ucnt#1\thanks{\MakeUppercase{#1}\futurelet\@tempa\@ucnta}
\makeatother

The difference here is that \uppercase is a TeX primitive and can only case change 'native' input for the engine, whereas \MakeUppercase is set up to deal with a range of additional macro-based entries, including \l.