addons: LetsEncrypt cannot obtain certificate with RFC2136 Server (HA 0.113.2)

Hi,

I am trying to use LetsEncrypt to get a certificate for a private domain behind a DNS server that I control. I set up RFC2136 dynamic update but when I configure the LetsEncrypt plugin I get an error. The log says:

[s6-init] making user provided files available at /var/run/s6/etc...exited 0.
[s6-init] ensuring user provided files have correct perms...exited 0.
[fix-attrs.d] applying ownership & permissions fixes...
[fix-attrs.d] done.
[cont-init.d] executing container initialization scripts...
[cont-init.d] file-structure.sh: executing... 
[cont-init.d] file-structure.sh: exited 0.
[cont-init.d] done.
[services.d] starting services
[services.d] done.
[12:05:40] INFO: Selected DNS Provider: dns-rfc2136
[12:05:40] INFO: Use propagation seconds: 60
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator dns-rfc2136, Installer None
Obtaining a new certificate
Performing the following challenges:
dns-01 challenge for name.dom.ain
Cleaning up challenges
Encountered exception during recovery: 
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/certbot/_internal/auth_handler.py", line 70, in handle_authorizations
    resps = self.auth.perform(achalls)
  File "/usr/local/lib/python3.7/site-packages/certbot/plugins/dns_common.py", line 57, in perform
    self._perform(domain, validation_domain_name, validation)
  File "/usr/local/lib/python3.7/site-packages/certbot_dns_rfc2136/_internal/dns_rfc2136.py", line 76, in _perform
    self._get_rfc2136_client().add_txt_record(validation_name, validation, self.ttl)
  File "/usr/local/lib/python3.7/site-packages/certbot_dns_rfc2136/_internal/dns_rfc2136.py", line 83, in _get_rfc2136_client
    int(self.credentials.conf('port') or self.PORT),
ValueError: invalid literal for int() with base 10: 'null'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/certbot/_internal/error_handler.py", line 125, in _call_registered
    self.funcs[-1]()
  File "/usr/local/lib/python3.7/site-packages/certbot/_internal/auth_handler.py", line 243, in _cleanup_challenges
    self.auth.cleanup(achalls)
  File "/usr/local/lib/python3.7/site-packages/certbot/plugins/dns_common.py", line 76, in cleanup
    self._cleanup(domain, validation_domain_name, validation)
  File "/usr/local/lib/python3.7/site-packages/certbot_dns_rfc2136/_internal/dns_rfc2136.py", line 79, in _cleanup
    self._get_rfc2136_client().del_txt_record(validation_name, validation)
  File "/usr/local/lib/python3.7/site-packages/certbot_dns_rfc2136/_internal/dns_rfc2136.py", line 83, in _get_rfc2136_client
    int(self.credentials.conf('port') or self.PORT),
ValueError: invalid literal for int() with base 10: 'null'
An unexpected error occurred:
ValueError: invalid literal for int() with base 10: 'null'
Please see the logfiles in /var/log/letsencrypt for more details.
[cont-finish.d] executing container finish scripts...
[cont-finish.d] done.
[s6-finish] waiting for services.
[s6-finish] sending all processes the TERM signal.

The configuration I am using says:

email: me@dom.ain
domains:
  - name.dom.ain
certfile: fullchain.pem
keyfile: privkey.pem
challenge: dns
dns:
  provider: dns-rfc2136
  rfc2136_server: dns.server.name
  rfc2136_name: TheName
  rfc2136_secret: >-
    PART1
    PART2
  rfc2136_algorithm: HMAC-SHA512

Do I NEED to supply the rfc2136_port setting? Does it not use the default? Also, where can I find /var/log/letsencrypt – and is there a way to run a --dry-run?