Email notification with attachment configured fails: Attachment file URL is not valid for: <file attachment name.pdf>

Greetings,

I have an O365 Nintex workflow that has a Send An Email action configured to send out an email with a PDF attachment from a document library from within the same site. The workflow gets terminated once it is kicked off with the following error:

Steps I took to configure the Send An Email action.

1.  Added email recipients and subject line information.
2. Set Email Service to "External email and attachment"
3. Check the Email Attachments checkbox, pasted the document URL in the URL field and clicked on "Include Attachment" button. File Name appears below the "Include Attachment" button.
4. Added the contents of the body of the email and saved the Send an Email action.

I am not sure if anyone else ran into this issue and found a fix for this or whether support can chime to see if I am doing something wrong.

As a side note, when I disable the Email Attachment checkbox, the workflow completes successfully but with no email attachment.

Thank you in advance for your time.