ClassCastException thrown by UserManager.getAllGroups

java.lang.ClassCastException: java.util.ArrayList cannot be cast to java.util.Set
 	at com.atlassian.jira.user.util.DefaultUserManager.getAllGroups(DefaultUserManager.java:255)

    @Override
    public Set<Group> getAllGroups()
    {
        return (Set<Group>) getGroups();
    }

is incorrect. getGroups() return now ArrayList (when the conversion from what crowd returns is not needed)!
The workaround requires changing the code to use getGroups() and recompile the plugin.