Password that I enter won’t save. I am seeing a keychain error displayed.
Login and password that you enter in 2Do, such as the one for your Reminders account, are stored in your account's Keychain. If 2Do can't seem to remember your login or password, and keeps reverting back to an empty login / password, your keychain may be corrupt.
To fix this, please follow these steps:
1) Open Terminal.app (spotlight for it, it's usually under /Applications/Utilities)
2) Enter the following command and hit return:
rm -Rvf ~/Library/Preferences/ByHost/*
3) Restart your Mac