I know this is an old post, but I didn't see an answer for it. I ran into the same problem with the Additional Web Server 9.1 SP5. I had the same problem with NTA & APM reports failing with the same error messages. To resolve the issue, For APM: copy APM-OrionReportWriter.cfg & APM-OrionReportWriter.schema files from the primary NPM server to the additional Web Server. Both of these files are located in the root /Orion directory. For NTA: Check to make sure you have Netflow-OrioinReportWriter.cfg in the /Orion root directory of your additional Web Server. Copy if it is missing. Copy Netflow-OrionReportWriter.schema file from the /Orion/NetFlowTrafficAnalysis folder of the primary NPM server to the same directory of the additional Web Server.
The changes are immediate, no reboots required.
Unfortunately for me, I had already tried copying the schema file over to the additional web server. The .cfg file already exsisted, but I tried copying it over anyway. I'm still in the same boat. My original ticket for this issue got closed somehow. I opened another one 2/23 and support downloaded my diagnostic files from all three of our servers on 3/09, but I still haven't heard from support on a resolution. Thanks for the suggestions though.