Is the Agentname fixed to ONE Google-Account - and where to find / change it?
Yes, just check on your “Settings Page”. You will find an option to drop the actual connected GoogleAccount on the next login.
Can i still Login with my Username/Password?
Yes, you can as well.
Will new users have to use Google-Login?
With the new sign-up process - YES. Till then - NO.
Error while trying to connect your Account to the Google-Login?
Try from a PC, because in some edge cases, it just will not work from a mobile phone. Thanks