I am having the same problem with Win7 SP1 and Thunderbird 24.1 (imap accounts) since upgrading to the latest Avast. I have used Avast for 10+ years without problems

I get ’ the mail shield encountered a problem’ message but mail is being scanned ( I looked in the message source for the Avast tag). This only happens for me after resuming from hibernation, so the network is probably not yet re-established, could this be the cause of the problem ?

I agree with the others who say this message is not very helpful at indicating the cause or solution to the error.

Otherwise, thanks Avast team for all your hard work.