Upgrade SSRS from 2014 to 2016 causes web portal to fail in browser

Hi,

After testing an in-place upgrade on 2 different machines in our network running SQL 2014 Express with SSRS, I am having the same problem.

Upgrade install has no errors and appears to succeed, but after trying to browse to the web portal (http://localhost/reports) I get the following error in the browser:

The service is not available.
The report server has encountered a configuration error.

Have tried running the browser as administrator, this is also being executed with a domain admin account.

Also the log files from SSRS folder are showing the following errors:

library!ReportServer_0-1!968!06/06/2016-11:22:33:: i INFO: Call to ListChildrenAction(/, False).
library!ReportServer_0-1!968!06/06/2016-11:22:34:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.ServerConfigurationErrorException: AuthzInitializeContextFromSid: Win32 error: 1355, Microsoft.ReportingServices.Diagnostics.Utilities.ServerConfigurationErrorException: The report server has encountered a configuration error. ;

%28type%3D%27UniversalBrand%27%2Ckey%3D%27logo%27%29 - Response 200 - 0:00:00.8536523
Microsoft.ReportingServices.Portal.WebHost!library!1a!06/06/2016-11:35:19:: e ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.ServerConfigurationErrorException: AuthzInitializeContextFromSid: Win32 error: 1355, Microsoft.ReportingServices.Diagnostics.Utilities.ServerConfigurationErrorException: The report server has encountered a configuration error. ;
Microsoft.ReportingServices.Portal.WebHost!reportserverwebapp!1a!06/06/2016-11:35:19:: e ERROR: [sprry2wr]: OData exception occurred: Microsoft.ReportingServices.Diagnostics.Utilities.ServerConfigurationErrorException: The report server has encountered a configuration error. .

After doing some testing, I have found that the only way to get it working (less than ideal) is to set the service to execute as my domain account user instead of the "Virtual Service" account that is set by default. No idea why this is, as it worked perfectly in that setup on SQL 2014.

Any help is much appreciated, thanks.

Update:

I've found that if I leave the Service Account in SSRS Configuration Manager set to "Virtual Service Account" as it is by default, but then run Internet Explorer as a Domain Admin user, the web portal executes properly!

Chris