This error is caused by the presence of an illegal character (one that is not allowed in the submission) somewhere within the tax form, often in a comment box.
Take a look through your form and remove anything unusual (i.e. not letters or numbers or standard punctuation) – for the SA900 the ‘£’ sign can be a problem. Also check any attachment filenames and remove any ‘&’ characters.
Sometimes these characters are not visible and have been entered when cut and pasting text or importing data. If this is the case then the quickest way forward is to download a new copy of the form and to complete it manually without using cut and paste or importing data.