Failed to determine supplementary groups: Operation not permitted

I was running into the same issue. Googling I found this thread: https://bbs.archlinux.org/viewtopic.php?id=233035

The problem is with how the service is being started. If you specify the user/group in the unit file then you should start the service as a system service.

If you want to start the service as a user service then the User/Group is not needed and can be removed from the unit config. You simply start the service when logged in as the current user passing the --user flag to systemctl.

I haven't found a solution to my problem but placing the unit files in ~/.config/systemd/user and removing the user parameterization fixed the issue for me.