AWS RDS VPC Connection Elasticbeanstalk

I really tried hard...I have a RDS MySQL Instance. Its security group is configured with MyIP such that I can access it with MySQL Workbench without any problems. Now I add to the SG my Elasticbeanstalk instance SG just as I do successfully with another Elasticbeanstalk Instance and another RDS DB. In both cases I use the JDBC_CONNECTION_STRING parameter. I ran out of ideas why one beanstalk/RDS pair works fine while the other doesn't. I even deleted the RDS instance and created a new one but to no avail. When I have the server output the connection string it is correct. Using the other RDS instance (via the JDBC_CONNECTION_STRING parameter), the server connects successfully to the other DB (whose SG was added this S2 instance SG). The 2 connection strings only differ by the DB name. Any idea highly apprechiated!