Can't access music iPod 4th gen on macOS 15.4

I can't access music on a iPod 4th gen 20GB on my Mac running macOS 15.4. Symptoms are:

• iPod is mounted and shows on the Desktop – as normal.
• Double-click the iPod disk icon opens a Finder window – as normal.
• As normal, iPod appears twice in the window sidebar – both have an eject button
  • when selected, the bottom item shows three folders: Calendars, Contacts, Notes – as normal.
  • when selecting the upper item, the window a rotating busy symbol but, after a minute or two shows “The selected device could not be found.”.
• The iPod also appears in sidebar of the Apple Music app and when selected, following a delay of a minute or so also shows “The selected device could not be found.”.

Have tried the iPod in a different user account with the same result. Then, tried the iPod on a MacBook Air running macOS Monterey 12.7.6. Success. The Finder window used to manage storage on the iPod appeared immediately.

Makes me think there's a bug in macOS 15.4. Had no problems back in February. Can't think of a reason why iPod cannot be accessed on the Mac now and yet can be accessed on the MacBook. Has anyone else seen this ?