Yes, I have checked the settings. I have been using this particular PC with unchanged settings with Thunderbird 3.0.1 and Avast for some time. But after the latest Avast program update it stopped working. The email server is my own. So I have full control over that, and I have tested with different setups. Outlook works fine on the same computer and with the same IMAP account. Other PC's are using the same server without any problems. The only failure I have seen so far is with Windows 7 64-bit, Thunderbird 3.0.1 and the latest Avast version.
The only thing I can think about that changed about the same time as Avast was updated and IMAP stopped working was that I configured Outlook to use the same IMAP account. However, I can't see how that would affect Avast's IMAP filter in combination with Thunderbird.
I might do a test; Make a clean Windows 7 install in a VM with only Avast and Thunderbird.