I'm on a Debian Linux machine using Icedove for email. (Icedove is nearly identical to Thunderbird but renamed by Debian.) The Icedove app is old, so I installed a newer Thunderbird downloaded from the mozilla.org site. I copied the user profile (named xxxx.profile where xxxx are several random characters) from .mozilla-thunderbird (used by icedove) to .thunderbird (used by genuine thunderbird) along with profiles.ini
When I run Thunderbird, it fails to connnect to the imap server. When I run Icedove, it works fine, getting and sending emails.
I checked all the settings in the Account Settings for the server. Server name, port number, etc. It all matches, except for Icedove shows only a checkbox for "use secure authentication" while genuine Thunderbird has a drop down list offerering several choices: normal, encrypted, kerberos, NTLP, and "any secure method". The last one was selected.
Icedove is version 126.96.36.199, and Thunderbird is version 3.1.10.