Failed to Connect to the Target Database
Here is my case, I added a cluster database named primdb.example.com followed by the service name of itself in Cloud Control 12c, and which does not match the database name primdb.
This could become a problem when click links to other URL. This is because the target names do not match the database names. Let's see how we reproduce the problem.
On the database home page, you can see the cluster databases are primdb.example.com and standb.example.com respectively for data guard environment.
Enter the primdb.example.com home page.
Go to data guard administration page.
Try to click any link to primdb or standb. The error message will happen.
A connection to database XXX could not be acquired.
Or, meet another form of error message.
Failed to connect to target database. Regions that fetch real-time data from target database will not be displayed. Failed to get Monitoring Credential for target name XXX targetType rac_database setnameDBCredsMonitoring
The solution is to remove the original targets and re-add the cluster databases which match the database name (i.e. primdb and standb in this case). After re-adding, here shows the database home page.