I was not binding to specific adresses, but was probably a problem with a specific release of Java (Oracle Java maybe.) My distro’s Java was doing weird video things, but the Oracle version was not, but then it could not reach outside the local computer. Debugging logs showed that it tried IPv6 and failed, then quit trying instead of falling back to IPv4. Disabling IPv6 in the Java JRE configuration solved the issue, but set me on the path to “modernize” my network stack. In hindsight, it’s probably not something that I really have the time to take on right now.