Hi @karsten-meichsner_9039,
I haven't seen that error before, but based on the text ("ProxyAuthenticationRequired for layer"), I think that Clair is trying to download an external layer?
Some container image manifests (especially Windows, but not entirely) will point to a URL outside of the registry. This is often done for licensing reasons. What this means is that, Clair (or the docker client) downloads the layers from a url instead of ProGet.
I'm not familiar enough with container scanners (Clair) to know how they search for vulnerabilities; I believe it's done by looking at the packages installed on the system. Log4j is not a package installed on the system (I think), but a library used in some applications.
Cheers,
Steve