I tried opening a test report which just has a simple static data table and a pie chart. I get this error:
We couldn't connect to the Analysis Services server. Make sure you've entered the connection string correctly.
TimeFri Nov 10 2017 23:52:19 GMT-0500 (Eastern Standard Time)
Why is it even trying to use Analysis Services?
I deleted this test and re-uploaded the same file. Now it works. And when I go to "manage" it doesn't think that it is trying to use Analysis Services. So I'm gonna call this a BUG in the PBIRS upgrade.
It seems that the issue happens after you upgrade the power bi report server to October, if the reports have imported/embedded data, we must re-publish reports, see: Tips when upgrading from the August preview to the October release of Power BI Report Server.