After an update to a new DataCenter version, you tried to update the remaining nodes onto a new clent software (GUI->System->NodesManagement->[Update Nodes(s)]).
This action fails with the message
Query "SELECT n FROM Node n WHERE (LOWER(n.hostname) = LOWER(:hostname) AND LOWER(n.domainname) = LOWER(:domainname)) AND (n.isDeleted = false OR n.isDeleted is null)" selected no result, but expected unique result.
The message appears in a popup window as well as in C:\programdata\novastor\datacenter\gui-client\gui.log.
Please proceed as follows:
Acknowledge the message popup window using button [OK].
Navigate to GUI->System->NodesManagement.
Mark entry for commandserver system and use context menu (right-mouse) Edit->ManageAddress.
Set Hostname to the FQDN, if it was a short name (or to short name, if it was FQDN).
Perform the update of the client version on the nodes.
Set Hostname back to the former value in Edit->ManageAddress.