Over the past weeks, we have had some issue with the amount of available disk space on our C drive. In order to free up some data, I have created a script which deleted used data from the C drive. One of the items it cleaned was the C:\Windows\Temp folder. After we ran the script on our servers, Excel Services suddenly stopped functioning properly.
As it turned out, Excel Services is writing some files into the C:\Windows\Temp folder on de Excel Services servers. After running the script, these folder were deleted, messing up Excel Services.
From within the Shared Services page (Excel Services Settings section > Edit Excel Services Settings > Workbook Cache Location), you can change the location to which Excel Services writes these temp files. When leaving empty, it will use the system Temp folder.
IMPORTANT NOTE: The application pool account needs to have write permissions to this folder. And an iisreset is required before the changed setting will be activated.