Obrigado ao Tom por me apontar na direção certa. Eu tinha adicionado uma entrada de DNS devido a alguns problemas de DNS um tempo atrás. Remover isso resolveu o problema. Aqui está o que Tom disse:
A captive portal usually works with DHCP and DNS. So Google-DNS might not know the hostname of the captive portal, which leads to an error. No hostnames and DNS are involved when browsing to an IP-address; so this worked. Setting the wireless connection to DHCP(yeah, also DNS) should work. When you want to use another DNS in your home-network, you should consider setting it on the access-point or router and not your local connection. – Tom 2 days ago