After spending time on this that I'd rather have back, I found that the root cause could have been any one or all of the widgets on the dashboard. After removing all the widgets on the source host and then redeploying the "All" configuration on the new host, the issue went away. The crash seemed to be triggered by clicking any link on the UI while the dashboard was present.
i've solved the problem. its very similar to bridge behavior i encountered in another installation. I only have vlans defined for my LAGG. once i created another interface that would be untagged on the LAGG, it picked up my native vlan as expected. all of the VIPs for the tagged interfaces started working.
so just for my own curiosity i deleted the native interface i crated and rebooted. everything still works. all in all i must have just jiggled the handle
Your statement that "pkg can't reach the pfsense servers" pointed me to the right direction; I haven't understood it fully, but I found a way at least to unlock the pkg issue.
In my case, it was due to a double stack IPv4/IPv6 issue: to solve it, I had to temporaly disable the network interface linked to the GIF port; removing IPv6 name resolution plus removing the IPv6 default gateway and firewall rules to route IPv6 traffic didn't suffice.
I don't like to be so inaccurate in test results, but as IPv6 connectivity was actually working, defining this problem will require some more tests and I meant to find a quick workaround for everybody experiencing this kind of issue.