I know that you stated that you are on Express Edition, but I wonder if this is your issue:

"HTTP 503: Service unavailable" error when you open the SSRS web portal after you upgrade to SSRS 2016

https://support.microsoft.com/en-us/kb/3171040

There is a fix for this in SQL Server 2016 RTM CU1. This should be applicable for Express Edition as well.

Cumulative Update #1 for SQL Server 2016 RTM

https://blogs.msdn.microsoft.com/sqlreleaseservices/cumulative-update-1-for-sql-server-2016-rtm/[/url]