OEM 12c Issue: Error "Failed to connect to target database. Regions that fetch real-time data from target database will not be displayed. Closed Connection" while connecting to RAC database through 12cR3 grid.

You may see this error "Failed to connect to target database. Regions that fetch real-time data from target database will not be displayed. Closed Connection" while navigating to a RAC database page in OEM 12cR3 grid.

This error is caused due to the Bug 17575631. 

You need to apply the patch for bug 17575631 to stop this error.

Oracle RAC: CRS-2412:The Cluster Time Synchronization Service detects that the local time is significantly different from the mean cluster time

You may see the alerts  "[ctssd(7700)]CRS-2412:The Cluster Time Synchronization Service detects that the local time is significantly different from the mean cluster time" regularly in your Grid Infrastructure alert log.

In that case check whether the clock is synchronized properly using Network Time Protocol (NTP) on all servers using below command

$ cluvfy comp clocksync -n all -verbose

....

....

Result: Clock synchronization check using Network Time Protocol(NTP) passed

Oracle Cluster Time Synchronization Services check passed

Verification of Clock Synchronization across the cluster nodes was successful.

As ntp is setup correctly as shown in above cluvfy output you can ignore this warning.

In 11.2.0.3 these messages will appear 2 per hour and in releases prior to 11.2.0.3 these messages will appear around 14 per hour.