Nginx responds with 502 on first submit

First entry submission results from an "empty" browser results in a 502 Bad Gateway response. Example query from Chrome:

curl 'https://dev.fevermap.net/api/v0/submit' -H 'Connection: keep-alive' -H 'Pragma: no-cache' -H 'Cache-Control: no-cache' -H 'Sec-Fetch-Dest: empty' -H 'User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/80.0.3987.132 Safari/537.36' -H 'DNT: 1' -H 'Content-Type: application/json' -H 'Accept: */*' -H 'Origin: https://dev.fevermap.net' -H 'Sec-Fetch-Site: same-origin' -H 'Sec-Fetch-Mode: cors' -H 'Referer: https://dev.fevermap.net/' -H 'Accept-Language: en-GB,en;q=0.9,sv;q=0.8,fi;q=0.7,en-US;q=0.6' --data-binary '{"device_id":1585077832156,"fever_status":null,"fever_temp":"39.3","birth_year":"1983","gender":"M","location_country_code":"FI","location_postal_code":"02700","location_lng":"24.7187657","location_lat":"60.2152143","symptom_difficult_to_breathe":true,"symptom_cough":true,"symptom_sore_throat":false,"symptom_muscle_pain":false,"diagnosed_covid19":false}' --compressed

Pressing submit a second time works, and subsequent submission also work fine.