There is early discussion at a WebmasterWorld thread with some frustrated Yahoo advertisers. Their frustration mostly stems from some of their imports into Yahoo Search Marketing. They say, the import feature doesn't always work.
Here is how one advertiser described it:
To be honest, I gave up on the upload feature in Yahoo. It has always been buggy for me, regardless if I down load my campaign directly from YSM, make changes and try upload the same, edited file. And forget about trying to upload my AdWords campaign into YSM! :)
Now, let me take Yahoo's side on this. As some of you know, I run a Web development company named RustyBrick. Part of that is to build out methods for clients to easily import and export data from their back office applications. Just like you would with importing and exporting into Yahoo.
Now, when you have users import files, if the structure of the document is changes or if there are invalid characters in the export, an error will be presented to the user on import. These changes to structure and characters can occur for dozens of reasons, even without the user knowing. That is where the error checking comes in. If an error is found, it is best to represent the error to the user as clear as possible. But there can literally be hundreds of different types of errors and causes for why the error might occur. As you find those outliers, they are programmed in. In addition, file sizes can impact the import as well.
There are many reasons why an import might fail on one day and work the next.
Over time, these imports will become more streamlined. Just keep reporting the exact error, submit your files to Yahoo and be descriptive. It will not only help you out in the future, it will help out other advertisers as well.
Forum discussion at WebmasterWorld.