The internet is down, except it is not

Monday 9 November, 08:30. Several users on floor 3 (VLAN 30, 10.30.0.0/24, gateway 10.30.0.1) say "the internet is down". Others on the same floor are fine. Over the weekend the DHCP scope for VLAN 30 was moved from the old Windows DHCP server (10.0.0.20, now switched off) to a new one (10.1.0.20). The company DNS servers are 10.1.0.53 and 10.1.0.54. Leases are 8 days.