Error reconfiguring organization - report server with empty response

I'm trying to move a report server for a CRM 2011 installation. The license for the current report server is expired as it was a trial, but I cannot seem to connect to the new report server. The report server is located on the SQL-server for the deployment. It is on-premise without IFD. I did not install the Report Server.

Whenever I try to edit the organzation through the deployment manager I get the following error

Setup failed to validate specified Reporting Services Report Server http://server3/reportserver. Error: Unknown exception thrown by report server
Client found response content type of '', but expected 'text/xml'.
The request failed with an empty response.

I also tried the FQDN and IP, nothing seems to work.

I can browse to http://server3/reportserver and http://server3/reports without problems (ReportServer/Reports/reportserver/reports does not matter).

The following error shows up in the event log when doing the same:

Web service request CreateRole to Report Server http://server3/reportserver/ReportService2005.asmx failed. Error: Client found response
 content type of '', but expected 'text/xml'.
The request failed with an empty response.

In the logs for the deployment manager I find the following:

21:56:51|  Error| Check ExistingRSValidator : Failure: Setup failed to validate specified Reporting Services Report Server 
http://server3/reportserver. Error: Unknown exception thrown by report server
Client found response content type of '', but expected 'text/xml'.
The request failed with an empty response.

The Report Service is run by a domain user account which is not a CRM user. Refering to this blog post I've added the user to the PrivUserGroup and the PrivReportingGroup. I have tried using the Network Service to run the Reporting Service, but ended up with errors on decryption of symmetric key.

There is a default web site located on the report server, but it does not matter if it is running or not. I can browse to the report server either way. I've tried adding SPN for the report server and the user account; setspn -a http/server3.fqdn domain\useraccount - but it did not have any effect. The Reporting Server is configured to authenticate using NTLM as the AuthenticationType in the config file is set to:

<RSWindowsNTLM />

Is there anyone who has seen something similar and can give me a solution that solves this??