Above message appears when trying to open server.

What can I do to fix this?

Thanks!

The last server pack containing the necessary files is 1.2.3 The newer ones aren't really:

All-of-Fabric-5-Server-1.2.3.zip    255796kb
All-of-Fabric-5-Server-1.3.0.zip    20kb

The new ones only contain start-scripts, icon and a yaml (which is used by some specific software?)

@Seegras

The last server pack containing the necessary files is 1.2.3 The newer ones aren't really:

This is intended, all required files should be downloaded on first launch of server.

@rivenluv

Above message appears when trying to open server.

What can I do to fix this?

You can edit server-setup-config.yaml and disable internet connection check under checkOffline: false
But make sure you do in fact have internet connection