I upgraded from version 14.0.600.398 to version 14.0.600.434 "Power BI Report Server - Evaluation". Existing reports have stopped working. I posted about that yesterday [http://community.powerbi.com/t5/Report-Server/Error-after-installing-latest-PBIRS/m-p/301233#M2154].
So today I made two new test reports. One uses SSAS Tabular and one uses Sql Server (both against Adventure Works sample).
Both of my new test reports were uploaded to PBIRS. Neither work in the PBIRS web portal. There is no error. The small view animation (circle of spinning dots) just keeps animating endlessly. The initial centered "Loading data..." animation finishes. It's the small one in the upper-left corner of the view that animates endlessly. This is true for both the SSAS and the Sql Server test reports.
Note that I did not upgrade Power BI Desktop. Perhaps that is necessary? If so, you should explicitly say so in the the download page at https://www.microsoft.com/en-us/download/details.aspx?id=56137
I had to reload or redeploy all the reports after upgrade from August 2017 preview to October 2017 GA version. Everything is working now after that. I do get "something went wrong" error after trying to upload large report with data (32 MB).
Have you tried to reboot your PBI Report Server service?