After a succesful change (no Internal Server Error page), the URL that loads is https://lists.fedoraproject.org/admin/lists/announce.lists.fedoraproject.org/ so perhaps that page failing to load quickly enough is the issue.
I can't reproduce it at the moment, and all I can find in the error log are messages such as End of script output before headers.
End of script output before headers
I might know what the cause is. Postorius, the admin UI, communicates with mailman via a REST API that the mailman processes expose. However, the embedded HTTP server is not multithreaded (at least in the version that we currently run). As a result, if this REST API is under load, the request are queued, and Postorius may end up waiting for Mailman to respond for long time, and apache will kill the request. I'm not sure weather Mailman now embeds a better HTTP server or if this is still an issue, but I've seen this sort of problems in the past.
Seems like a race condition, closing the ticket.
Metadata Update from @mohanboddu: - Issue close_status updated to: Will Not/Can Not fix - Issue status updated to: Closed (was: Open)
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