Thanks to [AZeroEight]) and fifh.
Due to a limited number of entries for ovelay icons in Microsoft Windows explorer, if a Dropbox sync-client was installed prior to Nextcloud’s, the sync -icons won’t show up in your local files.
In order to fool the explorer you need to rename the five registry key
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\ShellIconOverlayIdentifiers\OCxxxxx
in
HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\ShellIconOverlayIdentifiers\ OCxxxxx
(the spaces added, though one might suffice but i haven’t tried yet!)
If like Liberium, your using [OneDrive] too, the overlay keys number may may exceed the limit supported by the explorer and you may need another workaround…
(you could try to delete the five [OneDriveN] keys first …)