From the Oracle/Java documentation:

java.net.preferIPv4Stack (default: false) If IPv6 is available on the operating system the underlying native socket will be, by default, an IPv6 socket which lets applications connect to, and accept connections from, both IPv4 and IPv6 hosts. However, in the case an application would rather use IPv4 only sockets, then this property can be set to true. The implication is that it will not be possible for the application to communicate with IPv6 only hosts.

It's strange that things didn't work with this flag to false. It should be able to connect to both IPv4 and IPv6. Now that the author has set it to true, the drawback is that his IntelliJ won't be able to connect to IPv6-only hosts anymore.