It seems there are two significant parts in the log
C:\Program Files\easy-installer>"C:\Program Files\easy-installer\bin\adb\fastboot" devices 2>&1 | findstr fastboot
'findstr' is not recognized as an internal or external command,
operable program or batch file.
exit value = 255
This part is meant to detect that the phone has successfully rebooted to fastboot mode.
It fails because an internal comman (“findstr”) is not recognized.
@MichielP80 Which version of Windows does your computer run?
A little later there is
C:\Program Files\easy-installer>"C:\Program Files\easy-installer\bin\adb\adb" -s ""GS290CTLB02330"" reboot bootloader
error: device 'GS290CTLB02330' not found
C:\Program Files\easy-installer>if errorlevel 1 (exit /b 10 )
exit value = 10
Not 100% sure what happens here, but I guess the installer tries again to reboot the phone into fastboot mode because it failed to detect fastboot mode earlier. And as the phone likely is in fastboot mode already, “adb” commands will not work.
The conclusion: I’d say this is nothing that can be fixed by a user and most likely needs changes to the installer.
My best guess is that it’s not Windows 10 but some older version that does not contain the “findstr” command (I’m not too familiar with such Windows details as a Linux user, though).
I can later raise an issue on Gitlab for that.
@MichielP80 Would it be ok to copy the log file to the /e/ bugtracker? As far as I know, those kind of services where you can post text data like this log automatically delete them after a month or two.