Cloning worlds -> "World could NOT be cloned!"

[gmalone]

Chat: "World could NOT be cloned!"
Console: "[Multiverse-Core] Failed to copy files for world 'arena-badlands-clone', see the log info" -- there is no "log info"

This issue has been posted about for YEARS in many forums and as far as I can tell, no definitive answer. For the sake of discussion, assume that I am spelling correctly, and all the obvious stuff.

Source code for clone command: https://github.com/Multiverse/Multiverse-Core/blob/master/src/main/java/com/onarandombox/MultiverseCore/commands/CloneCommand.java

The high level call passage:
if (this.plugin.getMVWorldManager().cloneWorld(oldName, args.get(1))) { sender.sendMessage(ChatColor.GREEN + "World cloned!"); } else { sender.sendMessage(ChatColor.RED + "World could NOT be cloned!"); }

The actual processing of the cloning (called by above code) happens here at public boolean cloneWorld(String oldName, String newName):
https://github.com/Multiverse/Multiverse-Core/blob/master/src/main/java/com/onarandombox/MultiverseCore/utils/WorldManager.java

The things that can crash the cloning process include:

1. malformed command
2. the source world doesn't exist
3. the to-be-cloned world already exists
4. the folder copying fails
5. couldn't delete copied worlds' uuid.dat file

#'s 1,2,3 and 5 can be eliminated (superficially), which leaves #4, folder copying.

Dear dev, could you kindly advise what you know about this issue? It's been hanging out there a long time. Thanks. I'd be glad to help further.