Improper alphabetic constant with systeme

Question

I recently installed miktex in a new computer and I had to set everything up from the beginning. The following mwe is working just fine in my old computer.

\documentclass[a4paper,11pt]{article}
\usepackage[english,greek]{babel}
\usepackage[iso-8859-7]{inputenc}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage[T1]{fontenc}
\usepackage{txfontsb}
\usepackage[lite]{mtpro2}

%--------------------------
\usepackage{systeme,regexpatch}
\makeatletter
% change the definition of \sysdelim not to store `\left` and `\right`
\def\sysdelim#1#2{\def\SYS@delim@left{#1}\def\SYS@delim@right{#2}}
\sysdelim\{. % reinitialize
% patch the internal command to use
% \LEFTRIGHT<left delim><right delim>{<system>}
% instead of \left<left delim<system>\right<right delim>
\regexpatchcmd\SYS@systeme@iii
{\cB.\c{SYS@delim@left}(.*)\c{SYS@delim@right}\cE.}
{\c{SYS@MT@LEFTRIGHT}\cB\{\1\cE\}}
{}{}
\def\SYS@MT@LEFTRIGHT{%
    \expandafter\expandafter\expandafter\LEFTRIGHT
    \expandafter\SYS@delim@left\SYS@delim@right}
\makeatother
%----------------------------------------

\begin{document}

\[ \systeme{x,y,z} \]

\end{document}

Between the comment lines there is a patch for systeme package that I use in order to get curly braces using the mtpro2 package as shown here. It works just fine too in every document (in my old computer). But in the new installation I get the following error :

Improper alphabetic constant. {}{}

How do I fix this?