Here's the situation as it happened to me. I got a new router, and flashed OpenWrt over the manufacturer's factory firmware.
I connected to the login page (192.168.1.1) and tried to log in. But the web GUI simply goes back to the login page, as if my password were incorrect.
There's a longer description of what's happening in Github, (tl;dr - it's most likely a cookie problem), BUT...
The solution is to clear cookies for the router address (likely 192.168.1.1) and then refresh the login page. It should work.