On 11.08.17 00:00, Davide Perini wrote:
I'm pretty sure that the problem resides in your postfix/dovecot configuration, if you use those.
Sorry, but no. As I stated in my OP, other clients can access Dovecot and Postfix just fine (tested with iOS Mail, Apple Mail, Thunderbird on macOS and Windows). Roundcube 1.3 with PHP 5.6 is the only combination causing problems, and since Roundcube uses PHP to establish IMAP connections and I can connect fine if I disable PHP peer verification, the cause is definitely that verification mechanism failing on my Roundcube server. The crux is to figure out why it fails.
Google confirms that other people experienced similar problems after updating to PHP 5.6, which enabled peer verification as a default setting (see http://php.net/manual/en/migration56.openssl.php).
-Ralph