@StephaneDupont , That single rule prevents traffic from VLAN1 to VLAN2. It blocks traffic initiated from VLAN1 to any "server" in VLAN2 (the more intuitive behavior). But it also blocks traffic...

@Roro747 , I actually had checked that aspect already. TL-R605, max DHCP reservations? - Business Community (tp-link.com) That's a reply from staff... the limit was 1024 on the v1. I'd be surprised...

@Roro747 , Hmm, all your clients ended up in the 10.xxx range? None of them got 11.xxx addresses? And new ones now get 169.254.xxx addresses? Did you make any reservations that could get in the way?...

@StephaneDupont , While it would seem more natural to control inter-VLAN traffic at the gateway, GW ACLs are very coarse and don't handle your scenario. But switch ACLs might. They are less...

@Roro747 , BTW, you might want to upgrade that router. The current firmware version is 2.2.6...

@Roro747 , The range is 10.10 to 11.254... In other words, 10.10 to 10.254 and 11.1 to 11.254. That's nearly 500.