Okay got it now by pointing “root” to my “/etc/spreed” path, so the base path where the spreed server was extracted to. Thank you for your help. I will write some step-by-step guide about how to configure the autorun with creating the spreed user for it etc. for less experienced linux users 
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Actually which files/folders in the “root” directory need to be owned/readable/executable etc. by the spreed user? For security reasons I would just chown what is really necessary and leave as much as possible owned by root. I mean I don’t know how much is really possible through a system user without pw like this, but for in case ;).
€: Guide is done, I guess this topic could be closed as TO was already satisfied and further discussion about systemd unit/autorun should be made in guide topic: [Spreed.ME] Howto autorun "spreed-webrtc-server" step-by-step