I have just encountered a similar error and symptoms after deploying Power BI Report Server October 2017 GA on a SQL Server 2016 Enterprise server.  I had been running SSRS 2016 (and multiple versions before that) without issue.  The same URL and report have been working for years, so the only known change was PBIRS.  The error was occurring when clicking a URL link in a web page to run an SSRS/paginated report and generate it as a PDF so the user can save it to disk.  Based on other posts I found, I added "&rs:ClearSession=True" to the URL and it worked immediately just by refreshing the web page containing the URL (no server-side resets).  I've had to use that parameter before with SSRS reports embedded in an iFrame.