do you have a third party antivirus that might be scanning email. That is the sort of error I associate with them blocking the relevant file with their slow slow scanning. When you get an email Thunderbird opens the file and writes it to disk and them closes it again. The antivirus product then locks the file for up to 10 minutes while they scan. Except Thunderbird wants to write the next email about 1 second after the first and you get this sort of error. Personally I recommend excluding the entire Thunderbird [[Profiles - Where Thunderbird stores your messages and other user data|profile folder]] from any antivirus scanning.