Ah yes, that pesky bracket was in the wrong place! Just to be clear it should look like this.
@echo off
setlocal enabledelayedexpansion
set File_LOG_Mirror=EE26415780.txt
FOR /F "usebackq delims=" %%A in ("%File_LOG_Mirror%") do (
set Line=%%A
ECHO !Line:SAISON=PERIOD^+!>>"%File_LOG_Mirror%._temp"
)
DEL "%File_LOG_Mirror%"
REN "%File_LOG_Mirror%._temp" "%File_LOG_Mirror%"
exit /b
~bp