Never having used Avast One, I'm not speaking from personal experience, but I would probably gone for a clean install first removing Avast Free and a restart (possibly also using the uninstall utility) before installing Avast One.
Whilst the article doesn't specifically mention Thunderbird, it does mention:
Select your type of email account. If you do not use Outlook, select Other.
And:
If your email provider is not listed in the table above or you already set up your email account to work with Email Guardian, then continue with the steps below:
On the Avast One screen, enter your email address, then click Continue.
Another thought (from another post if my memory serves me well) is there could be two Avast entries in the Thunderbird Settings > Privacy & Security > Certificates (see attached). That person deleted both before reimporting the Avast Certificate. I don't know if they did a Thunderbird restart before reimporting the avast certificate.