If you see the following warning in the nexus.log relating to task "Automatically submit analytics events", then you need to add host name analytics.sonatype.com to SSL Certificates (Administration -> SSL Certificates)
2015-05-26 00:59:59 WARN [pxpool-1-thread-9] *TASK org.sonatype.scheduling.DefaultScheduledTask - Exception in call method of scheduled task Automatically submit analytics events com.sun.jersey.api.client.ClientHandlerException: javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
If your Nexus Repository instance is configured to use an HTTP proxy server that rewrites SSL certificates for secure (HTTPS) remote hosts, then please follow the resolution in article How to trust the SSL certificate issued by the HTTP proxy server in Nexus.