Please refer to my last post. I was whistling and I asked my DBAs to check the same db permissions in PRODUCTION and they did. And I whistled some more in happiness of resolving a long standing problem. And guess what, in PRODUCTION my steps did not resolve the problem. And I kept on getting:
No problem. I recommended the use of ULS Logs to pinpoint the problem and I did. Nothing. No Excel Service related error reported. No problem at all. I turned verbose logging on and started looking for relevant error. Nothing. A lot of errors, but no Excel Services Related error. Days passed. Nothing. Checked event viewers as thoroughly as possible. Nothing.
I was wrapping up the troubleshooting on one long afternoon. And was about to give up as I did in the last few days. As a final resort, before shutting my laptop, I checked the Global Settings in Excel Services Application Settings. And I was simply looking through. I did not know what the default settings are. So I opened the same corresponding settings in another SharePoint environment and started comparing. I was startled to see that we changed some global settings in PRODUCTION and they are not consistent.
Once I made those settings consistent, low and behold, it started working. In the image below are before and default settings.
Bottom-line, troubleshooting means keep on looking. And then look for some more.