When trying to start session receiving error message: "Application error occurred during the request processing, please try again"

When trying to run .NET Open SDK tests on Github using runner, installed testproject agent responds with message:

When trying to call manually POST http://localhost:8585/api/development/session, I am getting following error:
{“status”:500,“code”:500,“source”:null,“message”:“Application error occurred during the request processing, please try again”}

All works with my local agent but not one on the runner. And yes, both agents are registered…