Certificate problem

I have a new postfix/dovecot server. The configuration is more or less like the old one (upgrade from CentOS 6 to CentOS 8).

Since the upgrade K-9 Mail is not able to receive messages anymore from the server.
I am not sure, if the problem is in the configuration of my app (the imap client) or the configuration of the server.

How do I go about it?

Error in the app:
Zertifikatproblem (example.com)
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Log on the server:
Sep 15 20:54:53 localhost dovecot[1071908]: auth: Debug: auth client connected (pid=1071932)
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x10, ret=1: before SSL initialization
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: before SSL initialization
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2002, ret=-1: before SSL initialization
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: before SSL initialization
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: SSLv3/TLS read client hello
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: SSLv3/TLS write server hello
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: SSLv3/TLS write certificate
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: SSLv3/TLS write key exchange
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: SSLv3/TLS write server done
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2002, ret=-1: SSLv3/TLS write server done
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2002, ret=-1: SSLv3/TLS write server done
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2002, ret=-1: SSLv3/TLS write server done
Sep 15 20:54:53 localhost dovecot[1071908]: auth: Debug: auth client connected (pid=1071933)
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x10, ret=1: before SSL initialization
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: before SSL initialization
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2002, ret=-1: before SSL initialization
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2002, ret=-1: before SSL initialization
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: before SSL initialization
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: SSLv3/TLS read client hello
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: SSLv3/TLS write server hello
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: SSLv3/TLS write certificate
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: SSLv3/TLS write key exchange
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2001, ret=1: SSLv3/TLS write server done
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2002, ret=-1: SSLv3/TLS write server done
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2002, ret=-1: SSLv3/TLS write server done
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2002, ret=-1: SSLv3/TLS write server done
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2002, ret=-1: SSLv3/TLS write server done
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2002, ret=-1: SSLv3/TLS write server done
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL alert: where=0x4004, ret=558: fatal certificate unknown
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2002, ret=-1: error
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL alert: where=0x4004, ret=558: fatal certificate unknown
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL error: SSL_accept() failed: error:14094416:SSL routines:ssl3_read_bytes:sslv3 alert certificate unknown: SSL alert number 46
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL: where=0x2002, ret=-1: error
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Debug: SSL error: SSL_accept() failed: error:14094416:SSL routines:ssl3_read_bytes:sslv3 alert certificate unknown: SSL alert number 46
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Disconnected (no auth attempts in 0 secs): user=<>, rip=10.8.0.10, lip=XX.XXX.XXX.XXX, TLS handshaking: SSL_accept() failed: error:14094416:SSL routines:ssl3_read_bytes:sslv3 alert certificate unknown: SSL alert number 46, session=
Sep 15 20:54:53 localhost dovecot[1071908]: imap-login: Disconnected (no auth attempts in 0 secs): user=<>, rip=10.8.0.10, lip=XX.XXX.XXX.XXX, TLS handshaking: SSL_accept() failed: error:14094416:SSL routines:ssl3_read_bytes:sslv3 alert certificate unknown: SSL alert number 46, session=
Sep 15 20:54:58 localhost dovecot[1071908]: auth: Debug: auth client connected (pid=1071934)