I am having the same issue that a lot of your customers have been having.
When I log onto my primary email account, the following error message appears in a pop-up: "Your mail account username@comcast.net was disabled due to invalid credentials. Please edit the account and enter correct credentials to enable it again", where "username" is a fairly new secondary account. This has not happened with my older secondary accounts.
I am not able to access this account's email from my primary account, but I am able to log onto this account and send or receive email.
This error message is vague and unhelpful. From my primary account, I have changed the password on this secondary account, but that does not make any difference. I have deleted and re-added the account, but that does not make any difference.
Given how many other customers have been having the same issue and how long this issue has been happening, I would think you'd have a fix for this.
Using Windows 10 and the latest version of Firefox.
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