User doesn't exist in Scrive anymore
If the user who sent out the document doesn't exist in Scrive anymore, you won't be able to preview a document anymore because when fetching document previews, this is done directly from Scrive using the author credentials stored in the Salesforce user.
Since this user is no longer existing, this means that the credentials that are stored are also now invalid and unusable. For cases like these, administrators in Scrive can view the document by clicking "View in Scrive" which will open the Scrive e-Archive at scrive.com.
There is a known limitation in the browsers to the size of documents that can be previewed, this is about ~1.5MB in Chrome, but should be up to 4.5MB in Firefox.
Also keep in mind that the Scrive Document also increases in size with every signature.
So it might be visible from start but then after 1 - 2 signatures it may stop rendering in the view page. As a solution you can always see it in scrive.com and as an attachment after the pdf file is saved in SF database.
If it's critical that the document is preview-able in Salesforce one could download the PDF and then upload it as a file to the related record in Salesforce.
If you're still having issues you can contact the Scrive Support:
Tel: +46 8 557 66 167