Looks like the ImapListen= was blocking access ...
Just completely reinstalled Avast ; there is no ImapListen= in the new avast.ini and Avast now listens on port 12143 (default ?!).
But I can connect to my IMAP server again !!
edit : the defaults for the PopListen, SmtpListen and ImapListen lines now seem to be : 12110, 12025 and 12143 ?!
There are no 'Listen' lines in the newly created 4.5 avast.ini and the file is much smaller than the one 4.1 created ...