In an Oracle RAC environment, you must load SSH keys into memory for the terminal session where you start DBCA. If you do not do this, then you receive user equivalency errors when you attempt to start DBCA.
Use the following commands to load SSH keys:
$ exec /usr/bin/ssh-agent $SHELL $ /usr/bin/ssh-add
If needed, provide the pass phrase when prompted. You can then start DBCA.