Visio Services - The server failed to process the request

Another day, another interesting SharePoint error!  I finally got around to building out a Visio dashboard for my model office environment.  We've got a lot of servers out there and it's about time to get a handle on overall health and performance. 

The dashboard worked great until I let the team try it out - two folks reported back that the page was showing an error, "the server failed to process the request".  ULS showed a related error, at Microsoft.SharePoint.WebPartPages.Utility.SetThreadCulture - not totally obvious!

Searching out there turned up a blog post by Robert Seso of a similar problem on SharePoint 2010 - different ULS log entries, though.  I tried out the fix he suggested to solve his issue - grant a permission to the service account a used by Visio Services - not the SCOM service account or data reader account, but the service account configured for Visio Services in SP2013.  Here's the PowerShell to run:

$webapp = get-spwebapplication("http://mywebapp.myserver.com")
$webapp.GrantAccessToProcessIdentity("mydomain\sp_visiosvc")

Go back to your page, refresh, and so far so good.

http://www.robertseso.com/2012/02/troubleshooting-workflow-visualization.html

More clarification!  So far not so good.  I think the real issue here was on how I had set up the data source.  Initially, I had created a site collection, pointed the ODC file at that URL, then all was good.  I then decided to make things nicer and go with a host named site collection, which in & of itself was not the problem, the problem was, I used the original ODC file without updating it for the new HNSC.  I recreated the ODC file in the HNSC and recreated the Visio drawing to use this ODC file & it's been running fine since.