Hi Philippe Vitse,
Please follow the below steps to fix the issue in your end.
1. Please ensure to stop the Report Server using “Stop Syncfusion Report Server IIS Express” shortcut icon present in the desktop.
2. Delete the existing installed database and resources using the utility which is present in the below folder.
C:\Program Files (x86)\Syncfusion\Report Server\Utilities\DatabaseUninstaller\DatabaseUninstaller.exe
Please refer the below link for more details
3. Start the Report Server using “Start Syncfusion Report Server IIS Express” shortcut icon present in the desktop.
4. Now you need to do the Startup pages and login with Administrator account.
Thanks,
Arasuraja B