I was now able to resolve the issue. 
As far as I remember, I could fix it by following the following steps:
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create an A Record on my Windows 2016 DC DNS for http://ucs-sso-ng.dom.contoso.net/
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After this I was able to access the admin panel for http://ucs-sso-ng.dom.contoso.net/ , but still could not login, with either my admin nor the domain admin on the Windows server.
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I managed to resolve this by following the steps mentioned here again:
During installation I used the credentials of the domain admin on the Windows server and now can thankfully access the keycloak admin panel. In the consequence the join to the domain by keycloak was also completed without any issues