I also had faced the same issue. So to fix the issue I followed the below steps:
I just went to following path SSRS Configuration Manager > Encryption Keys > click Delete.
But when I did that I got following error message:
Failed to delete the encryption content in the report server database. Please execute the "DeleteEncryptedContent" stored procedure on the database manually.
After getting this error I went to following path SQL Server Mgmt Studio > Report Server > new query:
And executed the above command. After that I simply restarted the service.And it started working for me.
If you follow the above mentioned procedure carefully then your issue will also get resolved.