You can turn on debugging in admin but there is nothing to debug here.
You upload a file and the server drops the upload so there is nothing to process hense nothing to debug
It is clear Yahoo is dropping the upload server side cause none of your site's uploading tools work on the gallery or your forum attachments. Until they fix this issue there is nothing more I can suggest here.
The only thing I know about Yahoo is that there must be a tmp directory in the root of the website to allow uploads to flow. You can try deleting this directory and recreate it but other than that they need to answer why the server is dropping the file uploads.