music app library does not connect to local files on newer Mac (but does on the older one)

Hello friends

Apologies for the repeating question:

I want to upgrade from a 2013 Mac mini (running some older OS) to a 2018 Mac mini (running MacOS 14). either will be used as music-server.

All my music is ALAC from a hard disc.

The library is stored on that hard disc too (the whole "iTunes" folder is there...)

When I boot the 2013 Macmini and start playing music, all is well and the library points to all the files.

When I boot the 2018 Macmini and start music.app by double-clicking the same Music Library.musiclibrary, it doesn't see the files but wants to go to the cloud.

Since this Macs aren't connected to the internet (but by my phone's hot-spot), it is a bit difficult...

How do I get the app to recognize the files?

Many thanks!