Connect to Oracle RAC from SQL Developer?
Looks like the JDBC 11g NAT+RAC issue.

JDBC Connections Using SCAN Fail With ORA-12516/ORA-12520 (Doc ID 1555793.1)


  JDBC thin 11g connections using  Single Client Access Name (SCAN) 
  fail with errors:
  
  ORA-12520: TNS:listener could not find available handler for requested
  type of server 
  
  ORA-12516: TNS : listener could not find instance with
  matching protocol stack
  
  when client and database are on different networks and Network Address
  Translation (NAT) router is used for translation.  JDBC OCI and
  SQL*Plus connections work fine. JDBC connections using the HOST vip
  names work fine over the NAT router, only connections using SCAN fail.
  JDBC thin connections using SCAN without NAT router work without any
  problems


The above note suggests upgrading the JDBC driver to 12c.

Whenever you connect through SCAN, your connection is redirected. Previous versions of the Oracle JDBC driver can not handle that properly.

If you download the latest SQL Developer, that comes with the 12.1.0.2 JDBC driver.

Another solution that I prefer is to use OCI with an Oracle Client instead of JDBC (this makes possible to cancel long-running queries in SQL Developer). After installing the Oracle Client, you can configure this in Tools / Preferences / Database / Advanced.