Itunes, Synology NAS, setup

To start with be aware that there are not infrequent posts here about NAS problems with iTunes. NAS is great conceptually but in execution issues can arise with iTunes and have been doing so for more than a decade. Everybody points fingers at everybody else but the bottom line is you are always running a risk of a mangled library. Better is to use a plain external drive and use the NAS for backup.

I believe the server package pretends to be a shared iTunes library without actually having an active version of iTunes open at the time. Have you read the manual or online documentation for the NAS? I don't own one and what I know is from issues people post here.

The reason you were having problems before is your NAS is going to sleep and not waking up before you start iTunes. The same thing would happen even if you were using a plain external hard drive. If it only happens at startup you can try placing an alias to the iTunes application on the server, then an alias to that alias in your computer and start iTunes that way, not directly from the computer.