Thanks for getting in touch. It sounds like the problem is due to the process running the selenium tests (Tentacle) cannot interact with the browser to run the tests as it is running as a Windows service. You may have some luck by enabling the interact with desktop option.
Thanks for the fast reply.
I was really hoping this would fix our problems but it didn't seem to work.
For the moment we'll stick with the workaround of executing the service manually and in the future search more about this.