I found the culprit! The machine that I wanted to reach was a VM on the same machine as where the swag was installed and the network interface was not correctly configured: there was no communication possible between the host and the VM!
Linuxserver Discord pointed me in the right direction! Here is the clarification on openmediavault forum: ping vm through omv and viceversa? KVM plugin - General - openmediavault and here is how this should be solved: omv6:omv6_plugins:kvm [omv-extras.org]