The second step is to update your browser. If you are using Safari on a Mac, it is important to make sure that your operating system supports ProtonMail. The reason for this is that Safari disables certain technologies which are needed for it to function. By clearing the cache in your browser, the error should go away. You will be able to log in with the latest version of ProtonMail.

For more help - https://emailsdesk.com/protonmail-com-login/