If I go to Login - Piwik PRO and enter my account name wrong I do not get a error message, instead I get a page with an invalid SSL certificate.
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Could you please share more details with me? (what browser, what account, etc.) I was not able to reproduce the issue myself. Feel free to do so via private message if needed.
Go to Login - Piwik PRO
Put a random string in the account name field.
Press log in.
It works now, you have probably fixed it. (but the error message could be better, maybe check if the account exists before sending the user to a not existing site)