1) You may need to first clear your browser cookies/cache that could have previous (conflicting) login info stored. In your OS/system default web browser settings - usually Safari for iOS and Chrome for Android - clear your cookies and cache, then quite and restart the ReadCube app and try again.
2) For Android - if you have an older device with only a non-standard system web browser, you may need to install Chrome first in order to log in with the Google option.
3) If the login problem persists - in the ReadCube app at the login step, tap on the "Forgot your password" link and then enter the email address associated with your Google account. You will receive a password reset email to create a new password, which you can then use to log in to the app with the regular login form (on any ReadCube app or on the web).
If you've tried the above and are still experiencing problems, please contact us at firstname.lastname@example.org