Redirect on login fails in some cases

I don't quite get why yet, but in some cases, after logging in, the app stays on /login and shows a blank page with the logged-in navbar. This happened to me in this exact situation:

1. I started mongo, ICAT+ and the Data Portal at the same time.
2. ICAT+ failed to connect to mongo (probably because the container hadn't finished starting up yet.
3. The portal front-end opened up in Firefox and failed to sign-in (Sign in page with alert box).
4. I started ICAT+ again.
5. I clicked on "Sign in" => blank page with logged-in navbar and /login in the address bar.