To give an update here, we have investigated the issue within our organisation and the results are the following:
Issue is not related to VPN itself, but to rules applied by our organization (IPv6 is blocked)
Disconnecting VPN does not solve the issue because my organizaqtion also impose a Firewall when VPN is disconnected which is also blocking connection
The "bug" is know at Cisco side (tracked with https://bst.cisco.com/bugsearch/bug/CSCwf98780)
Workaround provided is to either: use and older version of Xcode or configure IPv6 tunneling to split exclude, with the local IPv6 addresses (FE80::/8) and the unique IPv6 local addresses (FD00:/8) among the tunnel exclusions
The change of rules is currently under investigation at my origanization but so far I'm still blocked from using Xcode 15. with iOS 17 devices