Everything else works fine. Postfix is receiving and delivery mail to Mailman3. List members are receiving the posts. I can access the list on Hyperkitty just no archived messages. What URL do you use to access HyperKitty.
I use http://mailman3-lists.com/mailman3/hyperkitty/. However I changed the base-url from localhost to mailman3-lists.com and saw the same error except the URL reflected mailman3-lists.com instead of localhost.
The problem URL above, http://localhost/mailman3/hyperkitty/api/mailman/archive is based on the base_url setting in mailman-hyperkitty.cfg which in your case is http://localhost/mailman3/hyperkitty/, byt the web server listening on localhost:80 is returning the Service Unavailable. It probably doesn't like the IP or name 'localhost'.
I have this in the apache2.conf file:
ServerName localhost
I can access the apache default page when using the following:
curl http://localhost.com
Set base_url in mailman-hyperkitty.cfg to use an acceptable host name instead of localhost or adjust your apache config to accept localhost.
Isn't ServerName enough?
This error also appears in the apache log file when new posts are sent to the list:
[Thu Apr 11 19:55:38.755750 2019] [:error] [pid 21370:tid 139988922513152] (32)Broken pipe: [client ::1:49574] sending data to httpd-UDS:0 failed [Thu Apr 11 19:55:38.769245 2019] [:error] [pid 21370:tid 139989049186048] (32)Broken pipe: [client ::1:49576] sending data to httpd-UDS:0 failed [Thu Apr 11 19:55:38.777583 2019] [:error] [pid 21370:tid 139988914120448] (32)Broken pipe: [client ::1:49578] sending data to httpd-UDS:0 failed [Thu Apr 11 19:55:38.787681 2019] [:error] [pid 21370:tid 139988905727744] (32)Broken pipe: [client ::1:49580] sending data to httpd-UDS:0 failed [Thu Apr 11 19:55:38.798273 2019] [:error] [pid 21369:tid 139989082756864] (32)Broken pipe: [client ::1:49582] sending data to httpd-UDS:0 failed [Thu Apr 11 19:55:38.807790 2019] [:error] [pid 21370:tid 139988897335040] (32)Broken pipe: [client ::1:49584] sending data to httpd-UDS:0 failed [Thu Apr 11 19:55:38.818123 2019] [:error] [pid 21369:tid 139989074364160] (32)Broken pipe: [client ::1:49586] sending data to httpd-UDS:0 failed [Thu Apr 11 19:55:38.827737 2019] [:error] [pid 21370:tid 139988888942336] (32)Broken pipe: [client ::1:49588] sending data to httpd-UDS:0 failed [Thu Apr 11 19:55:38.837612 2019] [:error] [pid 21370:tid 139988880549632] (32)Broken pipe: [client ::1:49590] sending data to httpd-UDS:0 failed [Thu Apr 11 19:55:38.848250 2019] [:error] [pid 21370:tid 139988872156928] (32)Broken pipe: [client ::1:49592] sending data to httpd-UDS:0 failed
Brian