For information. “Reflector Simple” seems to work without problems otherwise. What’s happening is that I decided to run it with “sudo” because it was going to ask for my password anyway for it to be able to save to “pacman” configuration.
[~]$ sudo reflector-simple [sudo] password for __: reflector-simple: info: reading file /etc/reflector-simple-tool.conf sed: -e expression #1, char 18: Invalid range end reflector-simple: info: reading file /etc/reflector-simple.conf
Then the GUI is shown and everything goes as usual.