I was configuring SharePoint and SQL Reporting Services and I kept getting the “SharePoint content service is null. Report Server may not have joined the SharePoint farm” error when trying to set the reporting service integration in SharePoint. No matter what I did I kept getting this error. I initially had this error using SQL Reporting Services 2008 SP1 and Windows SharePoint Services 3.0.
Thinking I had an issue with one or the other, I upgraded my SQL Reporting Services to SP2 and my Windows SharePoint Services to SharePoint Foundation 2010. The same error occurred.
Then something in my head clicked, stop and start the Reporting Service. Voila! This fixed the issue. So in the end, make sure when you change your Reporting Service mode from Native to Integrated, or vice-versa, you restart the reporting service.