[Updated 8 Sep 2021]

The Devs had resolved this issue. Do a hard refresh (ctrl +F5) on your browser and you should be able to see your cards and battle.

Many users are reporting a problem when opening the cards collection page or battling a match. This had been ongoing for days and seems like the Devs are still not resolving it. While waiting for them, there are a few ways you can do to bypass the problem.

The Problem

It is only affecting players in Southeast Asia and is likely caused by some CDN (AWS Cloudfront). The error that I am seeing is related to CORS, as below.

Because our browsers are preventing some files from being downloaded (due to the origin mismatched), we can't see our cards or play battle.

Solution
To bypass this, we can use a Chrome extension that temporary disable CORS checking.
https://chrome.google.com/webstore/detail/cors-unblock/lfhmikememgdcahcdlaciloancbhjino/related?hl=en
After installing, remember to click on the icon to unblock the checking. Like below

Now you will be able to see your cards and play battle. You can also disable the extension if you want, since the relevant files had already been downloaded in your cache.

This solution is relevant for Chrome. If you are using other browser, then you need to find extension for your browser.

Other solution
As we know the problem is affecting players in SEA. So by using a VPN and connecting to a server outside of SEA, that can solve it too. But not everyone has access to a VPN and some of us do not like using VPN.

Try these out and let me know if it works for you.