I have changed the default location for the open/save dialogs via
Tools # Options #Libre Office # Paths # My Documents.
This has the desired effect for the dialogs until I close the application.
After restarting the settings under Tools # Options #… still show my customized value, however it has no effect on the open/save dialogs.
These dialogs now again start with the libraries # documents folder.
Btw., it would be ok for me to disable the windows libraries concept on this computer completely if that helps.