This article is specifically about logging into the site. Any other pages, like the registration page, should work if you are logged in.
The short answer, caching. We've made some changes or our side to hopefully speed up the process. However, we are at the mercy of the internet overlords and their updating mechanisms. In theory, the problem will resolve itself in about a week.
What can we do for now?
Clearing your browser cache and cookies works for most people.
On your computer, this is the fastest way to clear the cache for a single page you are currently viewing (most browsers).
- Navigate to the website for which you want to clear the cache.
- Open the browser's developer tools by pressing
F12orCtrl + Shift + J(Windows/Linux) orCmd + Option + J(Mac). - Right-click the Refresh button in the browser's address bar.
- Select "Empty Cache and Hard Reload" from the context menu.
On your Android mobile device:
Click on the lock icon or the 
image next to the URL at the top.
Select Cookies and Site Data then click the trashcan icon.
On your iPhone (this is what Google says):
To delete cache & cookies for a specific website on your iPhone (Safari), go to Settings > Safari > Advanced > Website Data, find the site by searching or scrolling, then swipe left on it and tap Delete, or use the in-browser "Aa" icon for Website Settings to clear data for that site directly.