I just found out that if fancyhdr has to mention a chapter located in \backmatter, it does this by numbering it with the number of the last chapter.
For instance, this:
\documentclass[11pt, a4paper, oneside]{book}
\usepackage{fancyhdr}
\usepackage{lipsum}
\pagestyle{fancy}
\begin{document}
\mainmatter
\chapter{asd}
\lipsum
\chapter{asd2}
\lipsum
\chapter{asd3}
\lipsum
\backmatter
\chapter{conclusions}
\lipsum
\end{document}
ends up in having a "CHAPTER 3: CONCLUSIONS" written in the top-right part of the last page. First of all, this is the fourth chapter, but since we are in \backmatter the chapter shouldn't be numbered at all anyway.
So what I want to do is to correct this so that fancyhdr, just for the chapters in \backmatter, doesn't write that "CHAPTER X: ", and just writes the name of the chapter.
I found this other solution, but both the option proposed ends up in having chapter written in normal font, instead of caps. I tried to set everything in caps by using \MakeUppercase and \textsc in various places of the first code proposed, but I get errors everywhere, so I'm still wondering how to do that.
Thanks to anybody who'll help me ;)