itch.io

Here's what you can try.
In your browser, login to the EU portal with your itchio credentials.
If you're not able to login, then there's an issue on our side

Then launch the downloader and go through the authentication process, 
as you're already logged in it should be quite fast.

You might need to retry authentication in the downloader once or twice as wrong/outdated data are stored in your device and on our servers.