FWF-40 LAN connection to USW-48-PoE switch dead

I have a FWF40-3G4G (7.0.5 but upgraded today to 7.2.2) which has been working well for a few years now.

There is 1 LAN connection on port1 which goes to the main switch which was a Ubiquiti Lite switch. This switch has been replaced by a Ubiquiti USW-48-PoE.

As soon as we upgraded to the USW-48 there is no connection (i.e. no internet) from the FWF40 to the USW-.48 but all LAN traffic across the switch works.

When the FWF-40 is plugged into the switch all the port lights on the switch flash in unison (broadcast storm?) I also noted that the media sense lights are on at both ends.

So far I have tried the following:

1.Execute ping from Fortigate CLI to LAN devices plugged into switch - fails

2.Tried LAN port 2 on Fortigate (LAN1,2,3 are all on same virtual switch) - same issue

3.Tried a backup modem (Draytek) instead of FWF-40 - this works as expected (internet and passes traffic to LAN)

4.Took the FWF-40 to my lab and plugged it into a USW-24 - works as expected in so far as it can connect to any of the devices plugged into the USW-24

To Do:

a. Force PoE off on USW-48 for FWF-40 port

b. Factory reset FWF-40

c. Try port spanning on USW-48 on port 1 (FWF40) and port 5 (server) to see how far traffic is getting.

But since the FWF-40 seems to work with other switches, but other firewalls work with the switch - its hard to tell if the issue lies with the switch or the firewall?

Other suggestions?