The process of connecting your Gmail/Outlook 365 account involves a cross-subdomain cookie (*.myphoner.com) because we use custom subdomains for accounts.
This can result in error for some browsers if their security settings does not allow cookies to be shared across subdomains. The solution is to loosen up on that setting just for the duration of connecting your account, or to use another browser while connecting. Once your Gmail/Outlook 365 account is connected you can tighten security again.
In Chrome, go to your cookie settings and make sure it looks as follows:
Working cookie settings for Chrome
In Safari, open Preferences and go to Privacy and make sure it looks as follows:
Working cookie settings for Safari