Since we were hitting the limitation in number of ports for the Cascina domain (most probably we had already at least an event were port requests spilled over the allocated range), this morning we have expanded the range (as seen in the CmDomains file entry) from:
Cascina     cmns.virgo.infn.it     29000 29001 899 /virgoData/Cm
to:
Cascina     cmns.virgo.infn.it     28000 28001 1899 /virgoData/Cm
pushing down the start to 28000 and extending to 1899 ports.
This implyed a stop/restart of the NameServer wich went quite fast and smootly.
Checking the situation afterward with some processes restart / new processes start (including the restart of the VirgoOnline VPM instance) everything looked fine.