Jul 22 10:29:18 netresearch.de:saslauth info SASL EXTERNAL with cweiske.de failed Jul 22 10:29:18 s2sout8fa6a68 info sent dialback key on outgoing s2s stream Jul 22 10:29:18 s2sout8fa6a68 info outgoing s2s stream netresearch.de->cweiske.de closed: stream closed Jul 22 10:29:18 s2sout8fa6a68 info sending error replies for 1 queued stanzas because of failed outgoing connection to cweiske.de
netresearch.de had a SSL certificate for "xmpp.netresearch.de", which was wrong. After fixing the certificate, the connection works again.