With the discontinuation of support for OMV 3.x I finally upgraded to OMV 4.x (4..1.9) this week.
The update has fixed 
a problem with CIFS/SMB I had not realised was related to OMV. Until my OMV update, opening documents from OMV from within LibreOffice on Mint always suffered a 60-90 second delay. As the delay was restricted to that particular client application, it did not really occur to me that the problem related to OMV.
It is now merely of academic interest, but does anyone know what changes have been made to CIFS/SMB in OMV 4.x?
(Now if I can only work out why MyWebSQL won't connect to mysql in OMV 4, then my upgrade will be a complete success).