Some old, unsupported plugins cause Spark to freeze on exit (or on loading), e.g. whiteboard, editor (provided by version2software) or built-in reversi or tic-tac-toe games.

Error example:
Oct 04, 2016 8:00:05 PM org.jivesoftware.spark.util.log.Log error

SEVERE:

java.lang.NoSuchMethodError: org.jivesoftware.smack.provider.ProviderManager.getInstance()Lorg/jivesoftware/ smack/provider/ProviderManager;
at com.version2software.sparkplug.whiteboard.WhiteboardPlugin.initialize(Whiteboar dPlugin.java:84)

at org.jivesoftware.spark.PluginManager.lambda$initializePlugins$1(PluginManager.j ava:643)