Mailman REST API not available. Please start Mailman core.

Hello,

Upon clicking Domains I do get Mailman REST API not available. Please start Mailman core.

What is going on, would I like to summarize as

IPv6 request, IPv6 reject, IPv4 retry, IPv4 response not seen.

I came to that summary by sniffing port 8001 with tcpdump. A libpcap-file will be provided. (I do hope the gitlab web interface allows me to do so.)

Version information:

stappers@vincent:~
$ dpkg -l mailman3-web
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name           Version      Architecture Description
+++-==============-============-============-============================================================
ii  mailman3-web   0+20180916-8 all          Django project integrating Mailman3 Postorius and HyperKitty
stappers@vincent:~
$ cat /etc/debian_version 
10.11
stappers@vincent:~
$