VLAN network doesn't work with Tahoe

I have a UniFi network with a number of VLANs. Everything worked well until last night when I decided to upgrade my Mac (Mac Studio 2022) from Sequoia to Tahoe. I left the machine to get on with it while I went to bed.

When I woke up this morning I discovered that the Mac could no longer access my network. Restarting the Mac and restarting all the switches between the Mac and the firewall failed to fix the problem.

The machine obtains an IP address from DHCP without any problem:

% ifconfig en0
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    options=567<RXCSUM,TXCSUM,VLAN_MTU,TSO4,TSO6,AV,CHANNEL_IO>
    ether <MAC number>
    inet6 fe80::<64 bit IPv6>%en0 prefixlen 64 secured scopeid 0x13 
    inet 192.168.72.103 netmask 0xfffff800 broadcast 192.168.79.255
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect (2500Base-T <full-duplex>)
    status: active

The router of my VLAN is at 192.168.72.1. If I do arp -n 192.168.72.1 I see:

? (192.168.72.1) at <MAC number> on en1 ifscope [ethernet]
? (192.168.72.1) at <MAC number> on en0 ifscope [ethernet]

ping 192.168.72.1 gives me:

PING 192.168.72.1 (192.168.72.1): 56 data bytes
Request timeout for icmp_seq 0
Request timeout for icmp_seq 1
Request timeout for icmp_seq 2
Request timeout for icmp_seq 3
Request timeout for icmp_seq 4

--- 192.168.72.1 ping statistics ---
6 packets transmitted, 0 packets received, 100.0% packet loss

The port of the switch to which the Mac is connected is assigned to VLAN 72 (and always has been - it used to work).

I'm completely out of my depth: can anyone help me please?