Program: 34106
Project: 34106
Buggy source:
/** 
 * Checks if player can complete challenge
 * @param player
 * @param challenge
 * @return true if player can complete otherwise false
 */
protected boolean checkIfCanCompleteChallenge(final Player player,final String challenge){
  if (!players.challengeExists(player.getUniqueId(),challenge)) {
    player.sendMessage(ChatColor.RED + Locale.challengesunknownChallenge);
    return false;
  }
  if (!isLevelAvailable(player,plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str"))) {
    player.sendMessage(ChatColor.RED + Locale.challengesyouHaveNotUnlocked);
    return false;
  }
  if (players.checkChallenge(player.getUniqueId(),challenge) && !plugin.getChallengeConfig().getBoolean("String_Node_Str" + challenge + "String_Node_Str")) {
    player.sendMessage(ChatColor.RED + Locale.challengesnotRepeatable);
    return false;
  }
  if (players.checkChallenge(player.getUniqueId(),challenge) && plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str").equalsIgnoreCase("String_Node_Str")) {
    player.sendMessage(ChatColor.RED + Locale.challengesnotRepeatable);
    return false;
  }
  if (plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str").equalsIgnoreCase("String_Node_Str")) {
    if (!hasRequired(player,challenge,"String_Node_Str")) {
      player.sendMessage(ChatColor.RED + Locale.challengeserrorNotEnoughItems);
      player.sendMessage(ChatColor.RED + plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str"));
      return false;
    }
    return true;
  }
  if (plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str").equalsIgnoreCase("String_Node_Str")) {
    if (!plugin.playerIsOnIsland(player)) {
      player.sendMessage(ChatColor.RED + Locale.challengeserrorNotOnIsland);
      return false;
    }
    if (!hasRequired(player,challenge,"String_Node_Str")) {
      player.sendMessage(ChatColor.RED + Locale.challengeserrorNotCloseEnough);
      player.sendMessage(ChatColor.RED + plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str"));
      return false;
    }
    return true;
  }
  if (plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str").equalsIgnoreCase("String_Node_Str")) {
    if (players.getIslandLevel(player.getUniqueId()) >= plugin.getChallengeConfig().getInt("String_Node_Str" + challenge + "String_Node_Str")) {
      return true;
    }
    player.sendMessage(ChatColor.RED + Locale.challengeserrorIslandLevel.replace("String_Node_Str",String.valueOf(plugin.getChallengeConfig().getInt("String_Node_Str" + challenge + "String_Node_Str"))));
    return false;
  }
  player.sendMessage(ChatColor.RED + Locale.errorCommandNotReady);
  plugin.getLogger().severe("String_Node_Str" + challenge + "String_Node_Str"+ plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str"));
  plugin.getLogger().severe("String_Node_Str");
  return false;
}

Fixed version (ground truth):
/** 
 * Checks if player can complete challenge
 * @param player
 * @param challenge
 * @return true if player can complete otherwise false
 */
protected boolean checkIfCanCompleteChallenge(final Player player,final String challenge){
  if (!players.challengeExists(player.getUniqueId(),challenge)) {
    player.sendMessage(ChatColor.RED + Locale.challengesunknownChallenge);
    return false;
  }
  String level=plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str");
  if (!level.isEmpty()) {
    if (!isLevelAvailable(player,level)) {
      player.sendMessage(ChatColor.RED + Locale.challengesyouHaveNotUnlocked);
      return false;
    }
  }
  if (players.checkChallenge(player.getUniqueId(),challenge) && !plugin.getChallengeConfig().getBoolean("String_Node_Str" + challenge + "String_Node_Str")) {
    player.sendMessage(ChatColor.RED + Locale.challengesnotRepeatable);
    return false;
  }
  if (players.checkChallenge(player.getUniqueId(),challenge) && plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str").equalsIgnoreCase("String_Node_Str")) {
    player.sendMessage(ChatColor.RED + Locale.challengesnotRepeatable);
    return false;
  }
  if (plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str").equalsIgnoreCase("String_Node_Str")) {
    if (!hasRequired(player,challenge,"String_Node_Str")) {
      player.sendMessage(ChatColor.RED + Locale.challengeserrorNotEnoughItems);
      player.sendMessage(ChatColor.RED + plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str"));
      return false;
    }
    return true;
  }
  if (plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str").equalsIgnoreCase("String_Node_Str")) {
    if (!plugin.playerIsOnIsland(player)) {
      player.sendMessage(ChatColor.RED + Locale.challengeserrorNotOnIsland);
      return false;
    }
    if (!hasRequired(player,challenge,"String_Node_Str")) {
      player.sendMessage(ChatColor.RED + Locale.challengeserrorNotCloseEnough);
      player.sendMessage(ChatColor.RED + plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str"));
      return false;
    }
    return true;
  }
  if (plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str").equalsIgnoreCase("String_Node_Str")) {
    if (players.getIslandLevel(player.getUniqueId()) >= plugin.getChallengeConfig().getInt("String_Node_Str" + challenge + "String_Node_Str")) {
      return true;
    }
    player.sendMessage(ChatColor.RED + Locale.challengeserrorIslandLevel.replace("String_Node_Str",String.valueOf(plugin.getChallengeConfig().getInt("String_Node_Str" + challenge + "String_Node_Str"))));
    return false;
  }
  player.sendMessage(ChatColor.RED + Locale.errorCommandNotReady);
  plugin.getLogger().severe("String_Node_Str" + challenge + "String_Node_Str"+ plugin.getChallengeConfig().getString("String_Node_Str" + challenge + "String_Node_Str"));
  plugin.getLogger().severe("String_Node_Str");
  return false;
}