Faulty inadyn-mt.conf

I am having trouble with the inadyn-mt.conf file. A unit file:

[Unit]
Description=Internet Dynamic DNS Client
Documentation=https://sourceforge.net/projects/inadyn-mt/ 
ConditionPathExists=/etc/inadyn-mt/inadyn-mt.conf
After=network-online.target
Requires=network-online.target

[Service]
Type=simple
Restart=always
ExecStart=/usr/local/bin/inadyn-mt --input_file /etc/inadyn-mt/inadyn-mt.conf

User=inadyn
PrivateTmp=yes

[Install]
WantedBy=multi-user.target

starts inadyn. The config file:

$ ls /etc/inadyn-mt
640 -rw-r----- 1 inadyn inadyn 771 May  8 22:52 inadyn-mt.conf

looks like:

verbose                 
syslog
ip_server_name          icanhazip.com /

# update interval 10m
update_period_sec       600

# force update after 6h
forced_update_period    21600

# ipv6 priv pref = pub
addr_pref               pub


dyndns_system           default@freedns.afraid.org
username                

alias                   , ip4
alias                   , ip6


dyndns_system           default@duckdns.org
username                

alias

Starting the unit fails:

May 08 23:00:56 raspberrypi systemd: Started Internet Dynamic DNS Client.
May 08 23:00:56 raspberrypi inadyn-mt: Wed May  8 23:00:56 2019: W:LANG: Cannot open language file.  Will use english defaults, or default override (--lang_file ...)
May 08 23:00:56 raspberrypi inadyn-mt: Wed May  8 23:00:56 2019: S:INADYN: Could not open default servers_additional.cfg.  Will try svr_add_cfg option, if set...
May 08 23:00:56 raspberrypi systemd: inadyn-mt.service: Service hold-off time over, scheduling restart.
May 08 23:00:56 raspberrypi systemd: Stopped Internet Dynamic DNS Client.
May 08 23:00:56 raspberrypi systemd: Started Internet Dynamic DNS Client.
May 08 23:00:56 raspberrypi inadyn-mt: Wed May  8 23:00:56 2019: W:LANG: Cannot open language file.  Will use english defaults, or default override (--lang_file ...)
May 08 23:00:56 raspberrypi systemd: inadyn-mt.service: Service hold-off time over, scheduling restart.
May 08 23:00:56 raspberrypi systemd: Stopped Internet Dynamic DNS Client.
May 08 23:00:56 raspberrypi systemd: Started Internet Dynamic DNS Client.
May 08 23:00:56 raspberrypi inadyn-mt: Wed May  8 23:00:56 2019: W:LANG: Cannot open language file.  Will use english defaults, or default override (--lang_file ...)
May 08 23:00:56 raspberrypi inadyn-mt: Wed May  8 23:00:56 2019: S:INADYN: Could not open default servers_additional.cfg.  Will try svr_add_cfg option, if set...
May 08 23:00:57 raspberrypi systemd: inadyn-mt.service: Service hold-off time over, scheduling restart.
May 08 23:00:57 raspberrypi systemd: Stopped Internet Dynamic DNS Client.
May 08 23:00:57 raspberrypi systemd: Started Internet Dynamic DNS Client.
May 08 23:00:57 raspberrypi inadyn-mt: Wed May  8 23:00:57 2019: W:LANG: Cannot open language file.  Will use english defaults, or default override (--lang_file ...)
May 08 23:00:57 raspberrypi systemd: inadyn-mt.service: Service hold-off time over, scheduling restart.
May 08 23:00:57 raspberrypi systemd: Stopped Internet Dynamic DNS Client.
May 08 23:00:57 raspberrypi systemd: Started Internet Dynamic DNS Client.
May 08 23:00:57 raspberrypi inadyn-mt: Wed May  8 23:00:57 2019: W:LANG: Cannot open language file.  Will use english defaults, or default override (--lang_file ...)
May 08 23:00:57 raspberrypi inadyn-mt: Wed May  8 23:00:57 2019: S:INADYN: Could not open default servers_additional.cfg.  Will try svr_add_cfg option, if set...
May 08 23:00:57 raspberrypi systemd: inadyn-mt.service: Service hold-off time over, scheduling restart.
May 08 23:00:57 raspberrypi systemd: Stopped Internet Dynamic DNS Client.
May 08 23:00:57 raspberrypi systemd: inadyn-mt.service: Start request repeated too quickly.
May 08 23:00:57 raspberrypi systemd: Failed to start Internet Dynamic DNS Client.
May 08 23:00:57 raspberrypi systemd: inadyn-mt.service: Unit entered failed state.
May 08 23:00:57 raspberrypi systemd: inadyn-mt.service: Failed with result 'start-limit-hit'.

While, and this is surprising, starting inadyn manually works:

$ sudo inadyn-mt --input_file /etc/inadyn-mt/inadyn-mt.conf
Wed May  8 23:04:55 2019: S:INADYN: Started 'inadyn-mt version 02.28.10_audible' - dynamic DNS updater.
Wed May  8 23:04:55 2019: E:CACHE_LIST Failed opening cache file in ip_cache_list_read...
Wed May  8 23:04:55 2019: W:WAVE_UTIL: Error loading shared library, libao.so...
Wed May  8 23:04:55 2019: W:WAVE: Error loading shared library, libao...Install libao for audible network status alerts...
Wed May  8 23:04:58 2019: W:INADYN: IP address for alias ':ip4' needs update to ''...
Wed May  8 23:04:58 2019: W:INADYN: IP address for alias ':ip6' needs update to ''...
Wed May  8 23:04:59 2019: W:INADYN: Alias '' to IP '' updated successfully.
Wed May  8 23:04:59 2019: W:INADYN: DYNDNS Server response:
HTTP/1.1 200 OK
Server: nginx
Date: Wed, 08 May 2019 21:04:59 GMT
Content-Type: text/plain; charset=utf-8
Connection: close
Vary: Accept-Encoding
Cache-Control: no-store, no-cache, must-revalidate
Cache-Control: post-check=0, pre-check=0
Pragma: no-cache
Expires: Mon, 26 Jul 1997 05:00:00 GMT
X-Cache: EXPIRED

Updated  to  in 0.017 seconds

Wed May  8 23:05:01 2019: W:INADYN: Alias '' to IP '' updated successfully.
Wed May  8 23:05:01 2019: W:INADYN: DYNDNS Server response:
HTTP/1.1 200 OK
Server: nginx
Date: Wed, 08 May 2019 21:05:01 GMT
Content-Type: text/plain; charset=utf-8
Connection: close
Vary: Accept-Encoding
Cache-Control: no-store, no-cache, must-revalidate
Cache-Control: post-check=0, pre-check=0
Pragma: no-cache
Expires: Mon, 26 Jul 1997 05:00:00 GMT
X-Cache: MISS

Updated  to  in 0.017 seconds

Wed May  8 23:05:01 2019: W:INADYN: IP address for alias ':auto' needs update to ''...
Wed May  8 23:05:02 2019: W:INADYN: Alias '' to IP '' updated successfully.
Wed May  8 23:05:02 2019: W:INADYN: DYNDNS Server response:
HTTP/1.1 200 OK
Date: Wed, 08 May 2019 21:05:02 GMT
Connection: close
Set-Cookie: ; Expires=Wed, 15 May 2019 21:05:02 GMT; Path=/
Server: nginx/1.4.6 (Ubuntu)
X-Clacks-Overhead: GNU Terry Pratchett
X-Frame-Options: DENY

OK

But if I remove the duckdns part:

verbose                 
syslog
ip_server_name          icanhazip.com /

# update interval 10m
update_period_sec       600

# force update after 6h
forced_update_period    21600

# ipv6 priv pref = pub
addr_pref               pub


dyndns_system           default@freedns.afraid.org
username                

alias                   , ip4
alias                   , ip6

The unit starts as well:

May 08 23:09:41 raspberrypi systemd: Started Internet Dynamic DNS Client.
May 08 23:09:41 raspberrypi inadyn-mt: Wed May  8 23:09:41 2019: W:LANG: Cannot open language file.  Will use english defaults, or default override (--lang_file ...)
May 08 23:09:41 raspberrypi inadyn-mt: Wed May  8 23:09:41 2019: S:INADYN: Could not open default servers_additional.cfg.  Will try svr_add_cfg option, if set...
May 08 23:09:41 raspberrypi inadyn-mt: Wed May  8 23:09:41 2019: W:INADYN: Could not open default wave file, extra/wav/alarm.wav.  Using default, ./alarm.wav...
May 08 23:09:41 raspberrypi inadyn-mt: Wed May  8 23:09:41 2019: S:INADYN: Started 'inadyn-mt version 02.28.10_audible' - dynamic DNS updater.
May 08 23:09:41 raspberrypi inadyn-mt: Wed May  8 23:09:41 2019: E:CACHE_LIST Failed opening cache file in ip_cache_list_read...
May 08 23:09:41 raspberrypi inadyn-mt: Wed May  8 23:09:41 2019: W:WAVE_UTIL: Error loading shared library, libao.so...
May 08 23:09:41 raspberrypi inadyn-mt: Wed May  8 23:09:41 2019: W:WAVE: Error loading shared library, libao...Install libao for audible network status alerts...
May 08 23:09:41 raspberrypi INADYN: Wed May  8 23:09:41 2019: E:CACHE_LIST Failed opening cache file in ip_cache_list_read...
May 08 23:09:41 raspberrypi INADYN: Wed May  8 23:09:41 2019: W:WAVE_UTIL: Error loading shared library, libao.so...
May 08 23:09:41 raspberrypi INADYN: Wed May  8 23:09:41 2019: W:WAVE: Error loading shared library, libao...Install libao for audible network status alerts...
May 08 23:09:43 raspberrypi INADYN: Wed May  8 23:09:43 2019: W:INADYN: IP address for alias ':ip4' needs update to ''...
May 08 23:09:43 raspberrypi inadyn-mt: Wed May  8 23:09:43 2019: W:INADYN: IP address for alias ':ip4' needs update to ''...
May 08 23:09:43 raspberrypi inadyn-mt: Wed May  8 23:09:43 2019: W:INADYN: IP address for alias ':ip6' needs update to ''...
May 08 23:09:43 raspberrypi INADYN: Wed May  8 23:09:43 2019: W:INADYN: IP address for alias ':ip6' needs update to ''...
May 08 23:09:44 raspberrypi INADYN: Wed May  8 23:09:44 2019: W:INADYN: Alias '' to IP '' updated successfully.
May 08 23:09:44 raspberrypi inadyn-mt: Wed May  8 23:09:44 2019: W:INADYN: Alias '' to IP '' updated successfully.
May 08 23:09:44 raspberrypi inadyn-mt: Wed May  8 23:09:44 2019: W:INADYN: DYNDNS Server response:
May 08 23:09:44 raspberrypi inadyn-mt: HTTP/1.1 200 OK
May 08 23:09:44 raspberrypi inadyn-mt: Server: nginx
May 08 23:09:44 raspberrypi inadyn-mt: Date: Wed, 08 May 2019 21:09:44 GMT
May 08 23:09:44 raspberrypi inadyn-mt: Content-Type: text/plain; charset=utf-8
May 08 23:09:44 raspberrypi inadyn-mt: Connection: close
May 08 23:09:44 raspberrypi inadyn-mt: Vary: Accept-Encoding
May 08 23:09:44 raspberrypi inadyn-mt: Cache-Control: no-store, no-cache, must-revalidate
May 08 23:09:44 raspberrypi inadyn-mt: Cache-Control: post-check=0, pre-check=0
May 08 23:09:44 raspberrypi inadyn-mt: Pragma: no-cache
May 08 23:09:44 raspberrypi inadyn-mt: Expires: Mon, 26 Jul 1997 05:00:00 GMT
May 08 23:09:44 raspberrypi inadyn-mt: X-Cache: EXPIRED
May 08 23:09:44 raspberrypi inadyn-mt: [1B blob data]
May 08 23:09:44 raspberrypi inadyn-mt: ERROR: Address  has not changed.
May 08 23:09:44 raspberrypi INADYN: [429B blob data]
May 08 23:09:46 raspberrypi INADYN: Wed May  8 23:09:46 2019: W:INADYN: Alias '' to IP '' updated successfully.
May 08 23:09:46 raspberrypi inadyn-mt: Wed May  8 23:09:46 2019: W:INADYN: Alias '' to IP '' updated successfully.
May 08 23:09:46 raspberrypi inadyn-mt: Wed May  8 23:09:46 2019: W:INADYN: DYNDNS Server response:
May 08 23:09:46 raspberrypi inadyn-mt: HTTP/1.1 200 OK
May 08 23:09:46 raspberrypi inadyn-mt: Server: nginx
May 08 23:09:46 raspberrypi inadyn-mt: Date: Wed, 08 May 2019 21:09:46 GMT
May 08 23:09:46 raspberrypi inadyn-mt: Content-Type: text/plain; charset=utf-8
May 08 23:09:46 raspberrypi inadyn-mt: Connection: close
May 08 23:09:46 raspberrypi inadyn-mt: Vary: Accept-Encoding
May 08 23:09:46 raspberrypi inadyn-mt: Cache-Control: no-store, no-cache, must-revalidate
May 08 23:09:46 raspberrypi inadyn-mt: Cache-Control: post-check=0, pre-check=0
May 08 23:09:46 raspberrypi inadyn-mt: Pragma: no-cache
May 08 23:09:46 raspberrypi inadyn-mt: Expires: Mon, 26 Jul 1997 05:00:00 GMT
May 08 23:09:46 raspberrypi inadyn-mt: X-Cache: EXPIRED
May 08 23:09:46 raspberrypi inadyn-mt: [1B blob data]
May 08 23:09:46 raspberrypi inadyn-mt: Updated  to  in 0.187 seconds
May 08 23:09:46 raspberrypi INADYN: [470B blob data]

I guess my config is faulty.