Thunderbird does not connect to Dovecot. Webmail is ok.

I set up my mail server with Postfix (MTA) and Dovecot (IMAP, POP3). When I tested my installation with an open source webmail interface everthing has been working well: login with username and password, receive mails (port 993, ssl/tls) and send mails (port 587, starttls). Today I started testing local clients beginning with Thunderbird. Here I get an error message like this: Click Me.

In Dovecot I have already changed

ssl_protocols = !SSLv3 !SSLv2
ssl_protocols = TLSv1.2 TLSv1.1 TLSv1 !SSLv3 !SSLv2

Moreover I tried to deactivate the OCSP certificate validation in Thunderbird (Settings > Enhanced > Certificates > Validation)

But the issue keeps existing :( Anyone here to help me please?

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From the question linked here and other conversation on the internet, it seems as if there is a bug in Thunderbird. It looks like Thunderbird fails to prompt the user to accept a self-signed certificate.

One work around seems to be importing the certificate manually through Thunderbird’s certificate setting menu. You can find it through Preferences > Advanced > Certificates > View Certificates:

Import cert