I forced the Standard report to use this customized RDD and it worked fine… so, guess not the RDD.
Yes, I know to avoid the “Sync Dataset” like the plague. 
What’s so utterly confusing is why this works perfectly in Pilot, even after a fresh copy of the whole system and rpt files.
Do you have separate SSRS locations, based on which environment? We don’t. So any RDL I edit is used in both the Live and Test DB’s.
But if you do, then maybe a subreport is the problem. If a subreport uses a full URL for its location, then the parent report (residing on the URL for the live company), may be trying to access the RDL for a sub-report (that is on the URL for the Test company).
That’s a long shot, but worth checking out.
Got it working… whew!
I thought I found the issue to be the “default parameters”, which had a GUID pointing to the Pilot reports db. After changing, that didn’t fix it.
I then thought the issue was a dataset and subreport that Epicor’s uplift team built for us. I deleted the dataset and the subreport and the report ran without error, but this wasn’t ultimately the problem.
It seems the issue was due to some sort of cache on the SSRS server. Here are some observations:
With the subreport and dataset dropped, the same-named RDL file shrunk by a few KB (213KB to 211KB) (was this file size difference recognized as a new file, purging something?)
The non-working RDL file was renamed to “_bad”, added as a new style, and worked (thus, this report had nothing wrong with it)
I then deleted the missing subreport version via SSRS webview and renamed the “_bad” one to the base name… it worked!
In the end, the RDL and RDD file were never changed. The SSRS server simply freaked out somewhere and would not run the RDL file properly. It took a delete and rename to fix this.
I have ZERO idea why.
Thank you for all of the suggestions.
reporting issues are never fun, nice job finding out the issue