Hmm, that's weird... IIS URL Rewrite Module should've been installed with the Web Platform Installer...
It really shouldn't be this hard to get SilverStripe running, the process *should* be seamless on the installer, but variations in installed software and versions of Windows seem to make the process more complicated than it should be.
As far as I can tell, the error is about converting a varchar to datetime, and a value is out of bounds, if my translation of the error to English is correct.
I don't know why this conversion is happening though, and the query it tries to run seems fine to me.
Do you know which field and value it's failing on, maybe the error has more details in there than I can read?
Could it be that because SQL Server has German as the locale that it doesn't understand the date values given to it, so it thinks they're out of bounds? Seems kind of unlikely though...
EDIT: If you change the language to English of SQL Server, what happens?
You can do that using SQL Server Management Studio. Once you've got it open, right click the server on the left hand panel and go to Advanced and change the language field there.
If you can't find SQL Server Management Studio, you can install it through the Web Platform Installer, by the way.
Alternatively, if that doesn't work, find where SilverStripe is installed, go to the mssql/code/MSSQLDatabase.php file, find the two lines:
$this->query('SET QUOTED_IDENTIFIER ON');
$this->query('SET TEXTSIZE 2147483647');
and add this line above or below:
$this->query('SET LANGUAGE us_english');