It isn't displaying any error but I could not view my tasks or add through GMail integration.
Please clarify what happens exactly when you try to log in and at what point? You click on "login" and literally nothing happens? Or do you see a loading screen or a blank screen? Any details you could provide would be much appreciated.
In general, such issues can happen if your browser doesn't accept third-party cookies.
I can see a loading screen when I clicked on login. Then again the login screen displayed. It asking me for login again and again and again. :-(
Please tell me, did enabling third-party cookies in your browser solved this issue? If it did, you can then whitelist [*.]todoist.com
Enabling third-party cookies solved my problem.