I know this is an old thread, but I haven't found any newer ones. What has been recommended does not apply to the Synology version. There's no command-line parameters you can pass from DSM when starting the service.
Apparently, when you install the package and it asks for a username,it actually tries to create a new user on your NAS. If you install it with the default username, which is "admin" it will silently fail, as the "admin" user already exists on your system. The package still installs, but no user will have access to it.
Make sure to
enter a user account that isn't already in use when prompted during the installation. I recommend "tvheadend" as it's what the login prompt says anyway, and you won't have any collisions with existing users.