by Ken Task.
You've mentioned everything but web service ... apache? nginx?
Might look into:
Apache has a variable: LimitRequestBody
Default is usually unlimited.
0 (unlimited) to 2147483647 (2GB)
https://httpd.apache.org/docs/2.4/mod/core.html#limitrequestbody
Nginx
client_max_body_size
server block, and php block
'SoS', Ken