@dtc: Thanks for your feedback. Certainly someone else should confirm what I am reporting.
FYI - During testing described in Reply #48 Client 2 was offline. Tonight I messed around with Client 2 (both Server & Client 1 online), and for a second time the following summarized steps worked nicely:
1) Uninstall MC 29.0.74
2) Install MC 29.0.65 from downloaded installer
3) Update to MC 29.0.74
4) Restore backup from server, but ONLY Restore Library and Playlists to copy custom playlists and custom fields onto the client.
If I restore everything in Step 4), MC's DLNA network components (including Playing Now upper part on the Server and Client 1) become unpredictably tangled. The only sure way to undo the knots is to start all over again at Step 1) on Client 2. Then DLNA auto-magically corrects itself all around.
A major shortcoming is that custom Views are lost, and the View > Load / Save View menu comes up empty. Normally Views would be restored with the full settings (thankfully the left panel customization is restored by the above). Reading the forum, I think it used to be possible to manually copy custom View files from one install to another. However it looks like custom View files are inaccessible nowadays. I am guessing that View info is deeply integrated into the Windows registry (HKEY_CURRENT_USER > Software > J . River).
Windows updates which are sitting around waiting to be installed can badly jumble MC. The above steps were performed after installing the latest Windows updates this evening on all PCs:
2022-07 Cumulative Update for Windows 11 for x64-based Systems (Win 11 Pro KB5015882; Win 10 Pro & Home KB5015807).