From the documentation:
path: Required string attribute.
Specifies the file path or URL that is served in response to the HTTP error specified by the statusCode and subStatusCode attributes. If you choose the File response mode, you specify the path of the custom error page. If you choose the ExecuteURL response mode, the path has to be a server relative URL (for example, /404.htm). If you choose the Redirect response mode, you have to enter an absolute URL (for example, www.contoso.com/404.htm).
Try changing this line:
<error statusCode="500" subStatusCode="100" prefixLanguageFilePath="" path="/error.asp" responseMode="ExecuteURL" />
To this:
<error statusCode="500" subStatusCode="100" prefixLanguageFilePath="" path="/stadtbibliothek/error.asp" responseMode="ExecuteURL" />