I can see where they state that (in their response to you). My response was based on several others reporting issues with ecobee and 2FA within the last month. And removing 2FA solved their issue.
Here is one such thread:
Although, since you don't use 2FA, that can't be the problem you've run into.
Are you using the built-in ecobee integration or the one written by @storageanarchy?
Also tagging @mike.maxwell to let him know that ecobee has changed their API.
