HTTP to HTTPS - Authentification Windows pops up

I have used CERTIFY to get a domain certificate from Let's Encrypt.

I configured the URL rewrite - see web.config

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<system.webServer>
<defaultDocument>
<files>
<clear />
<add value="index.html" />
<add value="index.htm" />
<add value="index.php" />
<add value="Default.htm" />
<add value="Default.asp" />
<add value="iisstart.htm" />
<add value="default.aspx" />
</files>
</defaultDocument>
<rewrite>
<rules>
<rule name="Redirect to Https" enabled="true" stopProcessing="true">
<match url="(.*)" />
<conditions>
<add input="(HTTPS)" pattern="^OFF$" />
</conditions>
<action type="Redirect" url="https://{HTTPS_HOST}/{R:1}" redirectType="Temporary" />
</rule>
</rules>
</rewrite>
</system.webServer>
</configuration>

When I type https://www.akoscheminformatics.de I get a an authentification window. Even if I wanted I cannot log in with the server account. I host the server next to me.

Second, related sssue: If I use the URL rewrite rule "redirect to HTTPS" on forum.akoscheminformatics and typehttps://forum.akoscheminformatics.de I get the errror "Server not found"

Any idea where the problem might be?

Alex