Third floor users get 192.168.1.x addresses on Monday morning

Monday 08:40. The helpdesk gets a wave of calls from the third floor: some users cannot reach internal applications or the internet, while colleagues at the next desk are fine. The floor is VLAN 30, 10.30.0.0/24, gateway 10.30.0.1 on DSW1, which relays DHCP to the Windows server SRV1 at 10.10.50.20 with ip helper-address. The change calendar shows nothing over the weekend.