Android keeps asking "Sign-in to WiFi network" even if WPA/WPA2 is already authenticated
I'm trying to connect to a DLink Router protected with WPA/WPA2. It's our home router and I can connect perfectly to it from laptops and desktops, Linux and Windows alike. But when I connect to it using my Android phone (XPeria Z), I am always notified to "Sign-in to WiFi Network" after I've entered the WPA/WPA2 password. Tapping into the notification loads a 404 page from my router. Then, from my browser, trying to load any website just keeps looping to this 404 page. Other apps that connect to the internet do report connection problems.
Being that I'm not on a data plan and always connect via WiFi, I can't install any diagnostic tools unfortunately. FWIW, I've ensured that the connection to my router does not use WiFi Protected Set-up. I've also tried connecting to the router through an old Samsung Galaxy Y, with Android 2.x to same results.
I wonder if this is a problem with my phones or with my router?
It's a problem with your router. After Android successfully connects to a Wi-Fi network, it tests to see if it can use this network to access the Internet. This is because it's commonplace for Wi-Fi networks to redirect all requests to some kind of login page, until you've logged in. The "Sign-in to Wi-Fi network" notification is nothing to do with authenticating to the network itself: it appears when Android detects that the network is redirecting requests.
Given that you've said any page brings up this 404 error page from your router, that fits perfectly with the fact you're getting the notification. The fact that the router won't give you pages from the Internet is the root problem: it's nothing to do with WPA or anything like that. Does the 404 page tell you anything helpful?
Perhaps the router is set up with a list of devices (by MAC address), and the new device isn't on the list so the router won't give it Internet access.
Nope. Other devices don't have an issue. And nope, setting static IP's doesn't help. And nope it's not configured to discriminate on known MAC's. It redirects to connectivitycheck.gstatic.com (Google CDN like page for static content) and then fails, and requests a sign in again. In this case running 6.0.1 on a Motorolla XT910 using a custom Mowkee? Rom which never had the issue until recently.