While using (some versions of?) Java 8 to to run Openfire that is compilied with (some versions of?) Java 11, the following stacktrace is printed to standard out when a client tries to connect:
The client connection attempt silently files (time-out)
The problem is documented in greater detail in https://stackoverflow.com/a/61267496
The issue is that Openfire's build system does not perform sound cross compilation, hence generating "invalid" bytecode when used with newer Java runtime libraries. Smack ran into the same issue, which was fixed with SMACK-651.