1) Use a VPN
I recommend using a browser based VPN like in Built Opera GX VPN or on other browsers use Urban VPN https://www.urban-vpn.com/free-products/free-browser-extension/
If VPN doesn't work try changing the region of the VPN to Japan.
2) If above VPN does not work
I have a small work around .
Go here -> https://sdk-prod-cdn-aws.kurogame-service.com/pro/account-center/mc/home
->Login
->Link an Email
->And login with email+password ingame
Conclusion :-This issue has been fixed. If you still have it please report to Wuthering Waves Customer Support.