this is a an excellent example of why backups are just good practice. do you have a backup of the file? do you have the source for the file, in which you could create a new thumbnail from? have you tried deleting the corrupted file from the post, and then reuploading a new one? tbis will give you a new image id, and your attached item will no longer be throwing 404/403 errors.
a 404 error means the file is not found. it doesn't mean that the file is corrupt. a 403 is similar to a 404, but this time the server says basically "no access to whatever it is youre looking for."
this sounds like your server could have had a hiccup with the attached file. there are a few ways that could have happened, but its outside of ubbt's scope.
remove to offending image from your ubbt post. then reupload the image from a saved original/backup copy. this will regenerate the thumbnails you're asking for and will stop generating links to the old missing file.
