On Reddit /u/proghostbuster noted:
I’ve been having the same issue and did not have 2FA enabled. Had to change password but verified that it is letters/numbers only.
I tested IFTTT and it worked and connected with no issues. I noticed that the auth url’s were different from beestat and IFTTT.
IFTTT: https://auth.ecobee.com/u/login?state= beestat: https://www.ecobee.com/home/oauth/login.jsp?p=
I’m guessing, but IFTTT is probably using the newer authorization workflow that I am trying to get access to.