Unable to connect to imap/s, no meaningful error message

[lemmy04]

I'm trying to set up a mail account which uses imaps and smtps with a "private" certificate, i.e. issued by my own CA and not signed by any "public" CA.

Expected behavior

It should either work, or give me a meaningful warning (something like "SSL certificate issued by an unknown authority"), and preferrably should give me the option to look at details, and then decide to trust or not.

Actual behavior

"Unexpected error connecting to mail server", no further information.
And in the log files:
{"reqId":"aSo4fBjUT99VBddRK2r7","level":3,"time":"2021-03-16T07:23:38+00:00","remoteAddr":"2003:cf:9f3c:5e2e:6a05:caff:fe1b:695f","user":"mathias","app":"mail","method":"POST","url":"/index.php/apps/mail/api/accounts","message":"Creating account failed: Connection to IMAP at eregion.kicks-ass.net:993 failed. Error connecting to mail server.","userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:86.0) Gecko/20100101 Firefox/86.0","version":"21.0.0.18"}

which also didn't help.

As soon as I used the internal address which is also open for imap and smtp without SSL/TLS it worked... So it's most likely my untrusted certificate.

Mail app

1.9.3

Mailserver or service: (e.g. Outlook, Yahoo, Gmail, Exchange,...)
cyrus imap 2.4.19 on openSUSE 15.2

Server configuration

Operating system: openSUSE 15.2

Web server: apache 2.4.43

Database: mariadb 10.4.17

PHP version: (e.g. 7.0)

Nextcloud Version: nextcloud 21.0.0

Client configuration

Browser: Firefox 86.0

Operating system: Windows 10

[ChristophWurst]

Dup of #3006 and #2785