The installation guide and iXp interface suggest the Oracle jdbc connection string in the iXp Admin GUI should be:
Oracle 12c has introduced a new option called Oracle Multitenant which allows a multitenant container database to hold many pluggable databases. See here for more info: http://www.oracle.com/technetwork/database/multitenant/overview/index.html
If your AE Oracle 12c database is a pluggable database in a multitenant environment, the jdbc connection string in the iXp instance configuration needs to explicitly state the service name.
The issue is encountered because the connection string, while it does contain the SID, it does not explicitly state the service name with the necessary connection string flag SERVICE_NAME.