Nexus Repo gives 503 with Maven but not with browser

The problem is with your proxy settings. The current settings will cause all requests to localhost to go through the proxy which will not be able to connect with localhost and hence returns a 503.

To solve this issue add the following to the proxy settings in the settings.xml file:
localhost

Problem was related to the proxy...

When I configured the proxy in my Settings.xml it worked great for external sites, however, it screwed up internal sites (like my nexus repo). So I removed the proxy from my settings.xml, added it to my nexus server, and then mirrored the central repo to my nexus version of central repo.

Now everything works as expected...

I find this happens when I don't have proxies configured. If I add a broken proxy configuration that excludes everything I really access it works.

<proxies>
    <proxy>
        <active>true</active>
        <protocol>http</protocol>
        <host>broken.com</host>
        <port>3128</port>
        <nonProxyHosts>mymaven|myotherrepo:8080|foo|baa</nonProxyHosts>
    </proxy>
</proxies>

Presumably maven is taking default proxy configuration from somewhere, but I have not found out where.