Reporting service 2016 error

Hello,

We have server 2012 R2 with SQlL 2014 in it. Recently we upgraded to SQL 2016 and installed Reporting Services. We have a same test environment which is working fine but suddenly our production server failed. here is the Reporting Services log

Microsoft SQL Server Reporting Services Version 2015.0130.1601.05 ((SQL16_RTM).160429-2226)
English (United States)

C:\Program Files\Microsoft SQL Server\MSRS13.MSSQLSERVER\Reporting Services\Logfiles\ReportServerService__10_29_2016_19_45_02.log
ADxxxx
Microsoft Windows NT 6.2.9200
6.2.9200
11824
Hypervisor

AMD64
AMD64
rshost!rshost!878!10/29/2016-19:45:02:: i INFO: CLR runtime is initialized.
rshost!rshost!878!10/29/2016-19:45:02:: i INFO: Derived memory configuration based on physical memory as 503316020 KB
appdomainmanager!DefaultDomain!878!10/29/2016-19:45:02:: i INFO: Entered managed...