Unable to download pop3 email after latest engine update

My Thunderbird has been working fine with Avast for a few years until today. I updated the virus definitions and engine this afternoon, and now I am unable to download email from my pop3 server unless I disable Avast shields. Both Thunderbird and Mailwasher cannot download the email. It seems to me there is something wrong with this latest release of Avast update: 141015-6.
How do I restore the old engine?
Ted
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