Impossible to save files made by Apple programs on networkdisk, affects Pages Numbers Garageband and Keynote. Why?

Configuration: iMac (MacOs 10.15.7) connected with ethernet to Macmini (Catalina latest) with USB file disk.

Versions affected:

Pages: Latest

Numbers: Latest

Garageband: Latest

Keynote: Latest

Problem: Impossible to save files on networkdisk.

Apple related programs:

Files that are present on the networkdisk and are opened and changed can not be saved to that disk. Only duplicated.

The same files copied to the hosts desktop can be opened, altered and saved without problem.

Other programs:

Any other program (BBEdit, Affinity Designer, Phocus, Capture One 12) can create, change and save their files to this networkdisk

Why is it IMPOSSIBLE to save documents made by Pages, Numbers or Garageband and Keynote on a networkdisk?

Are there Apple personel present that read this and can answer me?

A solution is appreciated.

Thank you very much.