SP2010: Excel Services Error - Unable to Process the Request

I've been playing a lot with Excel Services for the last week and while it is nice, it is also temperamental. Most of this can be chalked up to inexperience on my part as I discover the closest to least privileges you can get for a SharePoint 2010 Excel Services Service Application, in light of the ever popular bug, "The workbook cannot be opened". However, I'm also convinced it's a little more particular than the previous version.

By default when I installed Excel Services using a PowerShell script, it had an entry for a Trusted File Location at "http://". After playing with the Business Intelligence Center template for a bit, I created my own workbook that had a PivotTable and a PivotChart that talked to my SSAS installation. Naturally I decided to update my Trusted File Location settings. So I deleted the default entry and created a more specific one that pointed directly to the Documents library that came with the template. Then I checked on the Excel example that came with the template and found that Excel Services was no longer working because it was "Unable to process the request".

Event Viewer showed nonstop critical errors and ULS had some gems in there pinning the blame on the Secure Store Service: "Request for security token failed with exception". I tried refreshing the key figuring that my WFE and App server were out of sync, but that didn't fix the problem. Lacking a better idea, I did an IIS reset on the WFE - and it started working again.

I'm not sure why it was unhappy, but at least it was only a 10 min troubleshooting span and a simple fix!