Cannot Change Service Account After SSRS Migration

Apologies if this is posted in the wrong place.

Following the migration of a Report Server Database to a new server, i have imported the SEK and the SSRS website appears to be working as expected, but i need to change the service account from the default.

When i try to change the user and enter a location and  a password for the Key file  it then asks for  administrator login credential for "applying grant rights script for the new windows service account", even though i am an administrator on the server and in SQL If i click OK i get a message saying 

Report Server Configuration Manager

A connection could not be established with the information provided Hit OK to change your connection settings  

A connection was successfully established with the server, but then an error occurred during the login process (Provider:SSSL Provider, error 0 - The certificate chain was issued by an authority that is not trusted)

Can anyone advise on how to resolve this?