Hello,
If you didn't find the solution, I faced the same issue and found that the Cluster upgrade profile still uses an old IP for the TFTP/FTP server (IP used for the last upgrade).
=> You can check this in : Managed Network node hierarchy (on the Cluster node), Configuration > System > Profiles. Under All Profiles, select Controller Profile > Upgrade.
For my upgrade, I reuse the old IP but probably you can clear it (and the method) to be able to use the new settings.