Some Ops Center users have run into an issue recently with certain Java versions on managed assets. Basically, if you upgrade to one of the problematic versions, it can disrupt communication between the different parts of Ops Center, causing assets to show as unreachable and jobs to fail.
The problem children are:
Java SE JDK and JRE 6 Update 101 or later
Java SE JDK and JRE 7 Update 85 or later
Java SE JDK and JRE 8 Update 51 or later
There's a simple workaround for this issue, which you can apply before or after you've hit the issue, which uses the OCDoctor script (by default, it's in the /var/opt/sun/xvm/OCDoctor/ directory).
On the EC, run the OCDoctor script with the --update option (the fix requires version 4.51 or later). Then run it again with the --troubleshoot and --fix options.
If you're using JDK 6, you then need to download a new Agent Controller bundle based on the version of Ops Center that you're using:
Enterprise Manager Ops Center 12.2.1:
Enterprise Manager Ops Center 12.2.2:
Enterprise Manager Ops Center 12.3.0:
Finally, use the same OCDoctor options (--update, then --troubleshoot --fix) on the Proxy Controllers. This will either clear up the issue, or prevent it from appearing at all.