Hello, this typically happens when there is an issue with the connection to the server. I would suggest clearing your client cache and flushing DNS. If the issue persists, please try restarting your pc and router, then you might need to login using a VPN, and using many different servers in the VPN.

On the upper righthand corner of the miniclient, there is a cogwheel looking icon, if you click on that, there will be an option stating "clean up system" this will clear your client cache.

The game does not actually need to load before you are able to clear the cache. Unless you are having issues with the login screen loading, in which case it would have to be an issue with a corrupted file, poor connection, or the game failing to load. These can be corrected by doing a few different things. First I would recommend restarting your pc and router, then if that does not work doing a fresh install of the client, uninstall and reinstall it, flushing DNS, or logging in via a VPN. With a VPN, you might be required to choose many different servers from within the VPN server to bypass the server that is causing the issue.