Hi,
No there is no firewall issue between the hosts and the vsm.
I solved the problem by installing the VXLAN VIB through VUM.
You can download the correct VIP on your vShield Manager: https://vsm-ip/bin/vdn/vibs/5.1/vxlan.zip
Import it into VUM as an extension and install it on your hosts.
No reboot required.
This solved the problem for me.
Thanks,
Peter