Yep, user error. "LogLevel info" was set in the vhost configuration but not the main Apache config, and this particular error shows up in the main server error_log rather than the vhost error_log
[Tue Sep 01 23:38:15.177559 2020] [wsgi:info] [pid 54042:tid 140644297517952] mod_wsgi (pid=54042): Starting process 'sgm-prod' with uid=50004207, gid=25 and threads=15.
[Tue Sep 01 23:38:15.183802 2020] [wsgi:info] [pid 54042:tid 140644297517952] mod_wsgi (pid=54042): Python home /apps/vmscan/.virtualenvs/sgm.
[Tue Sep 01 23:38:15.183862 2020] [wsgi:info] [pid 54042:tid 140644297517952] mod_wsgi (pid=54042): Initializing Python.
[Tue Sep 01 23:38:15.202600 2020] [wsgi:info] [pid 54042:tid 140644297517952] mod_wsgi (pid=54042): Attach interpreter ''.
[Tue Sep 01 23:38:15.204691 2020] [wsgi:info] [pid 54042:tid 140644297517952] mod_wsgi (pid=54042): Adding '/apps/www/sgm/wsgi/SGM' to path.
[Tue Sep 01 23:38:15.204792 2020] [wsgi:info] [pid 54042:tid 140644297517952] mod_wsgi (pid=54042): Adding '/apps/vmscan/.virtualenvs/sgm/lib/python3.8/site-packages' to path.
[Tue Sep 01 23:38:15.206152 2020] [wsgi:debug] [pid 54042:tid 140643793323776] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 13 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.206211 2020] [wsgi:debug] [pid 54042:tid 140643894036224] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 1 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.206233 2020] [wsgi:debug] [pid 54042:tid 140643868858112] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 4 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.206252 2020] [wsgi:debug] [pid 54042:tid 140643843680000] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 7 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.206270 2020] [wsgi:debug] [pid 54042:tid 140643818501888] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 10 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.206311 2020] [wsgi:debug] [pid 54042:tid 140643885643520] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 2 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.206350 2020] [wsgi:debug] [pid 54042:tid 140643877250816] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 3 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.206388 2020] [wsgi:debug] [pid 54042:tid 140643902428928] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 0 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.206602 2020] [wsgi:debug] [pid 54042:tid 140643860465408] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 5 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.206652 2020] [wsgi:debug] [pid 54042:tid 140643852072704] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 6 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.206702 2020] [wsgi:debug] [pid 54042:tid 140643835287296] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 8 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.206752 2020] [wsgi:debug] [pid 54042:tid 140643826894592] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 9 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.206799 2020] [wsgi:debug] [pid 54042:tid 140643810109184] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 11 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.206965 2020] [wsgi:debug] [pid 54042:tid 140643801716480] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 12 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:15.207067 2020] [wsgi:debug] [pid 54042:tid 140643784931072] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54042): Started thread 14 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:31.260173 2020] [wsgi:info] [pid 54042:tid 140643793323776] [remote 2001:420:1080:3004::ad25:5fd7:37700] mod_wsgi (pid=54042, process='sgm-prod', application=''): Loading Python script file '/apps/www/sgm/wsgi/SGM/run.py'.
[Tue Sep 01 23:38:34.044173 2020] [wsgi:error] [pid 53241:tid 140644082374400] [client 2001:420:1080:3004::ad25:5fd7:37702] Truncated or oversized response headers received from daemon process 'sgm-prod': /apps/www/sgm/wsgi/SGM/run.py
[Tue Sep 01 23:38:34.044202 2020] [wsgi:error] [pid 53241:tid 140644124337920] [client 2001:420:1080:3004::ad25:5fd7:37700] Truncated or oversized response headers received from daemon process 'sgm-prod': /apps/www/sgm/wsgi/SGM/run.py
[Tue Sep 01 23:38:34.198481 2020] [wsgi:info] [pid 52360:tid 140644297517952] mod_wsgi (pid=54042): Process 'sgm-prod' has died, deregister and restart it.
[Tue Sep 01 23:38:34.198534 2020] [wsgi:info] [pid 52360:tid 140644297517952] mod_wsgi (pid=54042): Process 'sgm-prod' terminated by signal 9
[Tue Sep 01 23:38:34.198566 2020] [wsgi:info] [pid 52360:tid 140644297517952] mod_wsgi (pid=54042): Process 'sgm-prod' has been deregistered and will no longer be monitored.
[Tue Sep 01 23:38:34.199931 2020] [wsgi:info] [pid 54088:tid 140644297517952] mod_wsgi (pid=54088): Starting process 'sgm-prod' with uid=50004207, gid=25 and threads=15.
[Tue Sep 01 23:38:34.204659 2020] [wsgi:info] [pid 54088:tid 140644297517952] mod_wsgi (pid=54088): Python home /apps/vmscan/.virtualenvs/sgm.
[Tue Sep 01 23:38:34.204707 2020] [wsgi:info] [pid 54088:tid 140644297517952] mod_wsgi (pid=54088): Initializing Python.
[Tue Sep 01 23:38:34.222980 2020] [wsgi:info] [pid 54088:tid 140644297517952] mod_wsgi (pid=54088): Attach interpreter ''.
[Tue Sep 01 23:38:34.225049 2020] [wsgi:info] [pid 54088:tid 140644297517952] mod_wsgi (pid=54088): Adding '/apps/www/sgm/wsgi/SGM' to path.
[Tue Sep 01 23:38:34.225148 2020] [wsgi:info] [pid 54088:tid 140644297517952] mod_wsgi (pid=54088): Adding '/apps/vmscan/.virtualenvs/sgm/lib/python3.8/site-packages' to path.
[Tue Sep 01 23:38:34.226427 2020] [wsgi:debug] [pid 54088:tid 140643877250816] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 3 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.226486 2020] [wsgi:debug] [pid 54088:tid 140643852072704] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 6 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.226511 2020] [wsgi:debug] [pid 54088:tid 140643826894592] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 9 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.226528 2020] [wsgi:debug] [pid 54088:tid 140643801716480] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 12 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.226546 2020] [wsgi:debug] [pid 54088:tid 140643902428928] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 0 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.226669 2020] [wsgi:debug] [pid 54088:tid 140643885643520] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 2 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.226715 2020] [wsgi:debug] [pid 54088:tid 140643894036224] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 1 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.226743 2020] [wsgi:debug] [pid 54088:tid 140643860465408] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 5 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.226767 2020] [wsgi:debug] [pid 54088:tid 140643868858112] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 4 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.226791 2020] [wsgi:debug] [pid 54088:tid 140643835287296] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 8 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.226814 2020] [wsgi:debug] [pid 54088:tid 140643843680000] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 7 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.227063 2020] [wsgi:debug] [pid 54088:tid 140643810109184] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 11 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.227117 2020] [wsgi:debug] [pid 54088:tid 140643818501888] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 10 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.227936 2020] [wsgi:debug] [pid 54088:tid 140643784931072] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 14 in daemon process 'sgm-prod'.
[Tue Sep 01 23:38:34.228030 2020] [wsgi:debug] [pid 54088:tid 140643793323776] src/server/mod_wsgi.c(9118): mod_wsgi (pid=54088): Started thread 13 in daemon process 'sgm-prod'.
And here's "vmstat 1" during that time. Free RAM is fine, and available cache RAM never drops below 53GB.
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
r b swpd free buff cache si so bi bo in cs us sy id wa st
0 0 990976 1296072 8044 55966592 0 0 15 53 1 1 12 3 85 0 0
0 0 990976 1295848 8044 55966592 0 0 0 4 311 398 0 0 100 0 0
0 0 990976 1295168 8044 55966592 0 0 0 0 4199 2938 2 2 97 0 0
1 0 990976 1268080 8044 55966592 0 0 0 0 4752 3559 9 4 87 0 0
1 0 990976 1225368 8044 55966712 0 0 0 0 2311 1228 33 2 65 0 0
1 0 990976 1139704 8044 55966712 0 0 0 0 2335 860 55 3 42 0 0
1 0 990976 1082504 8044 55966712 0 0 0 20 2862 626 35 3 62 0 0
2 0 990976 1013720 8044 55990828 0 0 0 0 2349 370 64 2 34 0 0
2 0 990976 969116 8044 55990828 0 0 0 186 2288 571 66 2 32 0 0
2 0 990976 925216 8044 55990828 0 0 0 0 2342 429 66 1 33 0 0
2 0 990976 917288 8044 55966712 0 0 0 0 3180 1632 43 4 53 0 0
2 0 990976 882840 8044 55966716 0 0 0 20 3742 2074 32 3 65 0 0
0 0 990976 875392 8044 55966724 0 0 0 44 8244 5675 18 5 77 0 0
1 0 990976 1062116 8044 55966744 0 0 32 0 3613 2515 31 3 66 0 0
0 0 990976 1290828 8044 55966864 0 0 0 320 2072 1016 31 1 68 0 0
0 0 990976 1290604 8044 55966864 0 0 0 740 731 822 0 1 99 0 0
0 0 990976 1290352 8044 55966864 0 0 0 0 563 635 0 0 100 0 0
0 0 990976 1289736 8044 55966864 0 0 0 0 796 894 0 0 100 0 0