When using the Zwyft or Zwyft Companion app on iOS, hitting TAB on your keyboard adds an extra character to the end of your username. This character isn’t visible, but is sent to the authentication server as part of your username. As a result, you won’t be able to login or, since the resulting username is a bogus email address, reset your password via the login screen.
This login pattern is extremely common: typing a username, hitting TAB, typing a password, hitting ENTER. Not supporting this in the login UI OR not stripping errant characters from the end of a valid email address should be considered a bug.
The “fix” is to not hit TAB to get to the password field, but use your touch screen to highlight the field instead.