Document Creation error - "We're sorry. We can't open document name because we found a problem with its contents"

Question

Morning Friends,
I have created a SharePoint 2010 "Site Workflow" that is designed to take information from a form and create a Word doc with the gathered information and store this Word doc in a document library.
I am using Sharepoint 2013 with Office 2013 
I understand there are a lot of steps (19) outlined below and I can provide more information as needed but the bottom line is this, workflow successfully takes info from an initiation form, uses the info to create a Word doc. Places this Word doc in a library.
When attempting to open / edit doc, receive error
"We're sorry. We can't open <document name> because we found a problem with its contents"
Details - No error detail available.
Any info or advice would be greatly appreciated. 
Very high level view of what I have done:
1 - Created content type called "Letters"
2 - Added site columns " First Name" and "Last Name"
3 -  Created and saved to my desktop a very basic Word document (Letter.docx) that says "Hello, my name is XXXX XXXX"
4 - In the advanced settings of the "Letters" content type I uploaded this "Letter.docx" file as the new document template.
5 - Created a new document library called "Letters"
6 - In Library Settings - Advanced Settings, clicked "Yes" to enable the management of content types.
7 - Then I clicked "Add from existing content types" and added the "Letters" content type
8 - Back in the advanced settings of the "Letters" content type I selected "Edit Template" and replaced the first XXXX with the Quick Part "First Name" and the second XXXX with the Quick part "Last Name"
9 - Created a new 2010 Site workflow called "Create a Letter"
10 - To the workflow I added the action "Create List Item"
11 - Configured the action to create Content Type ID "Letters" in the document library "Letter" 
12 - For the "Path and Name" I gave it a basic name of "Letter to"
13 - The next step was to create the Initiation Form Parameters and added to form entries "First Name" and "Last Name"
14 - I then linked the initiation form fields to the data source "Workflow Variables and Parameters" to their respective Field from Source parameters
15 - Went back to the "Path and Name" and modified the basic name of "Letter to" to include the first and last name parameters.
16 - Saved - published and ran the work flow.
17 - As expected, Initiation Form prompts for First and Last Name. ("John Doe") Then click "start
18 - Go to document library "Letters" and see a new Word document created titles "Letter to John Doe" 
19 - Go to open / edit the Word document and receive the following error
thoughts? Any info or advice would be greatly appreciated. 

Answer 1

See this MS support article for SP2010 workflows and generating Word docs:
https://support.microsoft.com/kb/2889634/en-us?wa=wsignin1.0
"This behavior is by design. The Create
List Item action in the SharePoint
2010 Workflow platform can't convert Word content type file templates to the correct .docx format."
I've had success in using SP 2013, Word 2013 (saving a .docx as the template instead of .dotx for the document library content type), and an SP 2010 workflow using SP Designer 2013.