This error will appear each time the user returns to the assignment submission as well as when an instructor attempts to grade an assignment due to the partial submission. This occurs when there are special characters in the file name attachment or the file name is longer than 26 characters.
The following are considered invalid characters in a file name: / : " ; < > # ? * |.
Make sure the file name does not contain special characters or spaces. Try renaming the file, and resubmitting.