The pop up window gives me the login, but it won't log in. It keeps coming back to the same login window.
Please make sure that your browser accepts third party cookies. You can also try to only whitelist [*].todoist.com
Thanks! It was the third party cookies.
This helped me as well, except if you want to only whitelist, it needs to be entered like this:
The period goes inside the bracket. Once I added that into the whitelist, it started working in Gmail.
worked for me!