You need to fill in a username is one error might be annoying for most users on SMF with thier new installation. This error in Windows IIS hosting is most likely due to security issues. In Linux, this error might not be visible.
To fix this issue, follow the simple steps given below:
1. Search for the ScriptURL in SMF forum and change it to relative address. This can be found in setting.php. Replace with absolute board URL with relative address. You can hardcode the relative path "/<script>"
2. The problem is with the absolute URL that makes the IIS to empty the values for all $_POST["fieldname"].
If you change the script URL to relative URL, then IIS would send the values to make the changes.