Hi Stefan,
Thanks for using Syncfusion products.
To do a clean uninstallation, you need to select the option “Remove resources and drop the database” in the uninstaller window as like below.
This will remove the resources – reports, data sources and files and will drop the database.
Please try uninstalling the Report Server with the above option selected and reinstall.
Now, you will be given option to choose database server and create admin user.
Thanks,
R. Venkatesan