The site sometimes allows two PDFs to be uploaded in the Lead Document section, which should only allow one file to be uploaded, therefore causing the error.
Remove the extra document from the filing, click Save, then click +Add Filing / +Add More to include the second document as a new Lead Document with its own separate filing code.
If a user receives this error and only one document has been uploaded, remove the filing entirely, save and exit the draft, then re-enter the draft and re-upload your file to clear the error.