Nexus: 404 on some artifacts

Something is wrong with Nexus:

There are missing artifacts in the Central proxy. E.g. this one:

Failed to execute goal on project windup-skiparchives: Could not resolve dependencies for project org.jboss.windup.quickstarts:windup-skiparchives:jar:2.0.0-SNAPSHOT: Failure to find org.apache.httpcomponents:httpcomponents-client:jar:4.3.4 in http://repository.jboss.org/nexus/content/groups/public/ was cached in the local repository, resolution will not be reattempted until the update interval of jboss-public-repository-group has elapsed or updates are forced -> [Help 1]

The respective directory listing lists various versions:
http://repository.jboss.org/nexus/content/groups/public/org/apache/httpcomponents/httpcomponents-client/

But the links there give 404:
https://repository.jboss.org/nexus/content/groups/public/org/apache/httpcomponents/httpcomponents-client/4.3.4/