XMPP not working (anymore)

[Azimoth]

Home Assistant release with the issue:
0.73

Last working Home Assistant release (if known):
unknown

Operating environment (Hass.io/Docker/Windows/etc.):
Debian 9.4 (Python virtual environment)

Component/platform:
https://www.home-assistant.io/components/notify.xmpp/

Description of problem:
Notifications cannot be sent anymore.

Problem-relevant configuration.yaml entries and (fill out even if it seems unimportant):

notify:
  - platform: xmpp
    name: recipient1
    sender: [email protected]
    password: !secret
    recipient: [email protected]

  - platform: xmpp
    name: recipient2
    sender: [email protected]
    password: !secret
    recipient: [email protected]

Traceback (if applicable):

2018-07-08 19:57:04 ERROR (read_thread) [sleekxmpp.xmlstream.resolver] All nameservers failed to answer the query _xmpp-client._tcp.my.duckdns.org. IN SRV: Server 2a02:908:2:a::1 UDP port 53 answered SERVFAIL; Server 2a02:908:2:b::1 UDP port 53 answered SERVFAIL; Server 192.168.x.x UDP port 53 answered SERVFAIL
Traceback (most recent call last):
  File "/srv/homeassistant/lib/python3.5/site-packages/sleekxmpp/xmlstream/resolver.py", line 289, in get_SRV
    dns.rdatatype.SRV)
  File "/srv/homeassistant/lib/python3.5/site-packages/dns/resolver.py", line 947, in query
    raise NoNameservers(request=request, errors=errors)
dns.resolver.NoNameservers: All nameservers failed to answer the query _xmpp-client._tcp.my.duckdns.org. IN SRV: Server 2a02:908:2:a::1 UDP port 53 answered SERVFAIL; Server 2a02:908:2:b::1 UDP port 53 answered SERVFAIL; Server 192.168.x.x UDP port 53 answered SERVFAIL
2018-07-08 19:57:04 ERROR (read_thread) [sleekxmpp.xmlstream.xmlstream] CERT: Invalid certificate trust chain.
2018-07-08 19:57:04 ERROR (read_thread) [sleekxmpp.xmlstream.xmlstream] Can not read from closed socket.

Additional information:
I am using LetsEncrypt certs and ca-certificates version 20161130+nmu1 is installed on the system. The XMPP server is Prosody version 0.10.2-1~bpo9+1.
It did work in the past, but stopped at some point a few months ago.

[Azimoth]

Found the solution: sleekxmpp doesn't support anything above TLS1 (I enforced TLS 1.1).
For more details see here.

[flowolf]

the TLSv1 issue is fixed with #17283
@fabaff please close