When certain valid and well known HTTP response codes are returned from a remote, they may end up Automatically blocking a proxy repository. Example nexus.log messages look like this:
2021-02-25 05:17:52,430-0800 INFO [qtp1606288645-338023] *UNKNOWN org.sonatype.nexus.repository.httpclient.internal.HttpClientFacetImpl - Repository status for docker-registry changed from AVAILABLE to AUTO_BLOCKED_UNAVAILABLE until 2021-02-25T05:18:32.430-08:00 - reason Unrecognized HTTP error for https://registry-1.docker.io
The problem is the actual HTTP response code is not logged there, only a static message "Unrecognized HTTP error" in place of a not present description of the status code.
The problem code generating the message does this:
Always include the HTTP status code as part of the "reason" logged. That way we at least have a clue of the type of response being returned from the remote.