Still no response from the Avast techs...
I noticed I use the same settings that are used by Waclaw66 in Thunderbird for my Outlook 2016: experience the same issues: IMAP disconnection, port settings, STARTTLS etc., are the same... I disabled Avast Mail Shields and then it's working fine.
So it looks like this is a general problem in version 19.1.2360.
For testing I also temporarily uninstalled Avast and installed Bitdefender Pro instead and then had no problems with Outlook. Don't get me misunderstood here: I'm still in favor of Avast and really do hope they're gonna fix this problem in the next update..