This method is just a guess, hope this works.
In short,
1. Export current avast certificate
2. Delete corrupted avast certificate from Thunderbird
3. Import current avast certificate into Thunderbird
Step by step method (strings might be wrong, as my Thunderbird is in Japanese)
- Open Avast Interface -> Settings
- Select "Active Protection" -> Mail Shield and click "Customize"
- Select "SSL scanning" and click "Export Certificate" then save it to desktop (or somewhere)
- Run Thunderbird and open Tools -> Options
- Select "Advanced" -> "Certificate" and click "Show Certificate" button
- Select "Certificate authority certificate"
- Find "avast! Web/Mail Shield root" then delete it
- Click "OK" twice to close "Options" window
- Follow step 5 to 6 again and import newly exported certificate at step 3
- Click "OK" twice to close "Options" window