What is the URL blocked error on login?

What you'll need

This error means the guest's device tried to reach a web address that isn't in the network's walled garden (the small list of domains a guest can reach before logging in) — anything outside that list is blocked until login completes, by design.
If guests report this on a URL you'd expect to work — assets for your splash page (the branded login page guests see when they join your WiFi), a social login provider, or your own redirect — the fix is usually to add that domain to your hardware's walled garden / allowlist. See Device connectivity FAQ for how that allowlist works alongside MyWiFi's login domain.
If it's happening on your own post-login redirect URL (configured on the Redirect tab), double-check that URL is entered correctly and is publicly reachable — see Add a redirect.
This is controlled entirely by your router or controller's own allowlist settings, not by anything in the MyWiFi campaign builder — MyWiFi only sets which domain your redirect points to, it doesn't manage the network-level allowlist.

Note: if a login method that used to work suddenly starts throwing this error, start with the walled garden/allowlist.