iCloud Reminders (CalDAV) sync says password is incorrect. I can’t log into 2Do.

Starting June 15th 2007, all users must use Two-Step authentication on their iCloud accounts for additional security if they wish to use their accounts in third party apps. Reminders sync (over CalDAV) will suddenly stop working for you after this date unless this is enabled. In case you don't enable two-step authentication, all 3rd party apps (including 2Do) using your account will stop working all of a sudden. You need to enable Two-Step authentication from your device Settings first and then create an app specific password to be used with 2Do.

There is no way around this. Using two-step was always highly recommended as prior to this lock down, rogue apps could potentially steal your passwords, photos, notes and do a lot more damage.
Please follow steps explained in the link below to setup an app-specific password for 2Do and use that from now on. This will apply to both 2Do on iOS and 2Do on Mac:
Using app-specific passwords with iCloud

Article ID: 647
Created On: Wed, Oct 1, 2014 at 9:27 PM
Last Updated On: Wed, Sep 13, 2017 at 7:52 PM

Online URL: https://www.2doapp.com/kb/article/icloud-reminders-caldav-sync-says-password-is-incorrect-i-can-t-log-into-2do.html