I'm still seeing these 502 errors every so often. They all appear to be related to image processing. More examples:
<address> - - [13/Jan/2020:10:45:23 -0500] "GET /forum/index.php?action=dlattach;topic=27033.0;attach=19331;image HTTP/1.1" 502 584 "https://www.mysite.org/forum/index.php?topic=27033.0" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36 Edge/18.18363"
<address> - - [13/Jan/2020:12:58:51 -0500] "GET /forum/index.php?action=dlattach;attach=8516;type=avatar HTTP/1.1" 502 584 "https://www.mysite.org/forum/index.php?topic=26983.0" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.88 Safari/537.36"
<address> - - [13/Jan/2020:14:10:43 -0500] "GET /forum/index.php?action=dlattach;topic=27033.0;attach=19337;image HTTP/1.1" 502 584 "https://www.mysite.org/forum/index.php?topic=27033.0" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.117 Safari/537.36"
<address> - - [13/Jan/2020:14:52:14 -0500] "GET /forum/index.php?action=dlattach;attach=10808;type=avatar HTTP/1.1" 502 584 "https://www.mysite.org/forum/index.php?topic=27028.0;topicseen" "Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.88 Safari/537.36"
<address> - - [13/Jan/2020:16:20:39 -0500] "GET /forum/index.php?action=dlattach;attach=15181;type=avatar HTTP/1.1" 502 182 "https://www.mysite.org/forum/index.php?topic=27028.0" "Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko"
<address> - - [13/Jan/2020:18:38:41 -0500] "GET /forum/index.php?action=dlattach;topic=27032.0;attach=19345;image HTTP/1.1" 502 584 "https://www.mysite.org/forum/index.php?topic=27032.0" "Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.117 Safari/537.36"
<address> - - [13/Jan/2020:19:27:10 -0500] "GET /forum/index.php?action=dlattach;topic=27058.0;attach=19393;image HTTP/1.1" 502 182 "https://www.mysite.org/forum/index.php?topic=27058.0" "Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko"
<address> - - [13/Jan/2020:22:00:47 -0500] "GET /forum/index.php?action=dlattach;topic=27044.0;attach=19355;image HTTP/1.1" 502 182 "https://www.mysite.org/forum/index.php?topic=27044.0" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15) AppleWebKit/605.1.15 (KHTML, like Gecko) FxiOS/21.0 Version/13.0.3 Safari/605.1.15"
<address> - - [14/Jan/2020:00:16:21 -0500] "GET /forum/index.php?action=dlattach;attach=10809;type=avatar HTTP/1.1" 502 584 "https://www.mysite.org/forum/index.php?topic=24730.105" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36 Edge/18.18363"
<address> - - [14/Jan/2020:00:16:21 -0500] "GET /forum/index.php?action=dlattach;topic=24730.0;attach=15630;image HTTP/1.1" 502 584 "https://www.mysite.org/forum/index.php?topic=24730.105" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.102 Safari/537.36 Edge/18.18363"
<address> - - [14/Jan/2020:09:56:33 -0500] "GET /forum/index.php?PHPSESSID=94u8rk5ttviq6fv7abnhu9rmdl&action=dlattach;topic=27058.0;attach=19389;image HTTP/1.1" 502 584 "https://www.mysite.org/forum/index.php?PHPSESSID=94u8rk5ttviq6fv7abnhu9rmdl&topic=27058.0" "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; Win64; x64; Trident/6.0; MDDCJS)"
Note the action passed to index.php: "action=dlattach", with each error associated with an avatar or an image attached to a topic. Is there a clue here?