SSRS and Managed Service Account - impossible to configure?
Yes you can run SSRS with a Managed Service Account. 



I would double check that your MSA is installed on your SSRS server. You can do so with Powershell.

Get-ADServiceAccount -identity msaname -properties hostcomputers |select hostcomputers


If your SSRS server is not listed, you will need to install the MSA on the server. This needs to be run locally (on SSRS server). You MUST be domain admin to run Install-ADServiceAccount successfully. 

Install-ADServiceAccount msaname