Hi, Richard,
I don't recall the details, so I could be way off base, but this
[Sun Aug 15 09:19:52.318582 2021] [core:notice] [pid 19706:tid 140315143705728] AH00094: Command line: '/usr/sbin/apache2' [Sun Aug 15 09:19:52.732381 2021] [proxy_http:error] [pid 19707:tid 140314912167680] (20014) Internal error (specific information not available): client 37.201.214.82:9292] AH01102: error reading status line from remote server 127.0.0.1:8000
indicates that Apache's (?) proxy_http server expects to find something that isn't there running on the same host on port 8000. Since it's not "refused connection" or similar, evidently it's connecting (presumably to Mailman core's REST API) but not getting the "status line" (greeting?) it expects. Now, for a proxy to know what status line to expect, either it's speaking a well-known protocol (such as SMTP or HTTP), or it's part of the same application suite. But Mailman is neither.
So I suspect you have your proxy configured to proxy Apache to Mailman instead of the other way round. Or maybe it's proxying from client 37.201.214.82 to Mailman, but it should be proxying from client to Apache. I guess the latter is more plausible, in the former there'd be some kind of triangular relationship among client, Apache, and Mailman, which doesn't fit the model of "proxy". Is either possible?
HTH, but if it doesn't ring a bell, say so and we'll try something else.
Steve