I think you should take the error as a warning. Yahoo are not capable of running a mail server, their product is buggy and prone to failure. I recently saw a article where Verizon has basically written billions off the value of it's yahoo division.

This will be another case of yahoo and their inscrutable errors and non existent support of acknowledgement of problems.

Is the archive folder within the yahoo mail account? The move command would indicate that it is and their server can't complete the move for an individual message to the new folder.

I have same problem on tampabay.rr.com. Just started a couple of days ago.

This article discusses some similar errors that occur while archiving Yahoo mail with a 3rd-party program. While this isn't identical with archiving through TB, it suggests that one workaround is to enable access by 'less-secure' apps on the Yahoo website. It's also recommended to archive large numbers of messages in small batches.