Did you delete and recreate the template after changing POSTORIUS_TEMPLATE_BASE_URL? If not, the template table still has the old URI.
Yes. Every test is: delete template and users (via Postorius), make any config changes, restart everything, then new template and new user (via Postorius).
What does
ps -fwwu mailman|grep runnershow?
# ps -fwwu mailman|grep runner mailman 375726 375716 0 02:00 ? 00:00:01 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=archive:0:1 mailman 375727 375716 0 02:00 ? 00:00:01 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=bounces:0:1 mailman 375728 375716 0 02:00 ? 00:00:01 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=command:0:1 mailman 375729 375716 0 02:00 ? 00:00:01 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=in:0:1 mailman 375730 375716 0 02:00 ? 00:00:01 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=lmtp:0:1 mailman 375731 375716 0 02:00 ? 00:00:01 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=nntp:0:1 mailman 375732 375716 0 02:00 ? 00:00:01 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=out:0:1 mailman 375733 375716 0 02:00 ? 00:00:01 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=pipeline:0:1 mailman 375734 375716 0 02:00 ? 00:00:01 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=rest:0:1 mailman 375735 375716 0 02:00 ? 00:00:00 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=retry:0:1 mailman 375736 375716 0 02:00 ? 00:00:00 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=task:0:1 mailman 375737 375716 0 02:00 ? 00:00:01 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=virgin:0:1 mailman 375738 375716 0 02:00 ? 00:00:01 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=digest:0:1 mailman 375790 375734 0 02:00 ? 00:00:00 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=rest:0:1 mailman 375791 375734 0 02:00 ? 00:00:00 /opt/mailman/venv/bin/python3 /opt/mailman/venv/bin/runner -C /etc/mailman3/mailman.cfg --runner=rest:0:1