The great thing about building out your own farm is ... you know how it was built. Sadly, if you take a new job, chances are you will be inheriting someone else's mess. Such was the case today :-(
A user was trying to use the Excel Web Access web part to display an Excel spreadsheet chart on their site. The web part was throwing a 'Unable to process the request ... Please try again ...' error. Checked Excel Calculation Services service application configuration in Central Administration, everything looked good. Digging through the ULS logs showed some interesting entries like
ExcelServerApp.ClearTempPath: Failed to create the Ecs temp path.
EngineInterop::LoadDll: Failed to load our engine (xlsrvol.dll)
To find these, I looked for 'Excel Services Application service is starting' and the errors would directly follow. For me, the fix was: