Thanks to all.
I figured the problem.
The “Application-Pool” cannot directly connect to the internet, because we have generally only connection
through proxies.
I did add following to the “web.config” of the Application-Pool.
<system.net>
<defaultProxy>
<proxy
proxyaddress=“http://yourproxy.yourdomain.local:8181”
bypassonlocal=“true”
/>
</defaultProxy>
</system.net>
And now everything works.