Hi,
I had a similar problem with Gmail/Thunderbird after AVAST8 was installed on my Windows 8 computer. Thunderbird would no longer download or send mail on my two Gmail accounts. I deleted the accounts from Thunderbird and tried to add them back. When I tried to add them Thunderbird gave a message “Configuration could not be verified – is the username/password wrong?” It wasn’t, because on my Android tablet I was able to send and receive on these accounts with the same username/password. This is what I tried:
1. I uninstalled and re-installed Thunderbird- same problem.
2. Using the guest account on my Windows computer, I opened Thunderbird and tried to add the same GMAIL account. This time I received a message to confirm a “security certificate exception”. When I examined the details of this I saw that certificate was associated with AVAST. After confirming the exception, I was able to send and receive mail in the Guest account.
3. I then found this thread and followed the various suggestions. I confirmed that when I disabled SSL checking in AVAST I could add the GMAIL account in Thunderbird and send and receive mail. But if I enabled it, I could not send or receive. I followed the procedure described in
http://www.avast.com/faq.php?article=AVKB91#artTitleThis did not fix the problem.
4. I concluded that it must be something in my TB Profile that was preventing Thunderbird from generating the “Confirm Security Certificate exception” dialogue. In my profile folder there was a file called cert8.db. With Thunderbird closed I deleted this file then re-opened Thunderbird. This time I was prompted with the “security certificate exception” dialogue box. I confirmed the exception and then was asked to enter my GMAIL username and password. My Gmail accounts now work fine.
So, in my case the solution was to delete the following file:
C:\Users\YourWindowsUsername\AppData\Roaming\Thunderbird\Profiles\YourProfile.default\cert8.db
Hope this may help others who have this problem.