What is the cause of the error message that appears when attempting to move items as they are being copied to a sub-folder but not being removed from the original folder?

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The most likely cause of this error message is that the file transfer is being interrupted or is incomplete. This can happen if the original folder is locked or in use by another program, or if there is a problem with the file transfer settings. Another possible cause is that there is not enough storage space available for the copied files in the destination folder.