The error log does not show anything, and the access log shows only one request during that:
<IPADDR> - - [14/Oct/2019:12:50:18 +0200] "POST /api/v1/message_send HTTP/1.0" 200 26 "https://support.whatsasolution.com/" "Mozilla/5.0 (X11; Linux x86_64; rv:69.0) Gecko/20100101 Firefox/69.0"
Even if I simulate the request on command line with curl, curl shows that:
> POST /api/v1/upload_caches/703429498 HTTP/1.1
> Host: support.whatsasolution.com
> User-Agent: curl/7.66.0
> Accept: */*
> Content-Length: 2886071
> Content-Type: application/x-www-form-urlencoded
> Expect: 100-continue
>
* Mark bundle as not supporting multiuse
< HTTP/1.1 413 Request Entity Too Large
< Server: nginx/1.10.3
< Date: Mon, 14 Oct 2019 10:51:56 GMT
< Content-Type: text/html
< Content-Length: 199
< Connection: close
<
<html>
<head><title>413 Request Entity Too Large</title></head>
<body bgcolor="white">
<center><h1>413 Request Entity Too Large</h1></center>
<hr><center>nginx/1.10.3</center>
</body>
</html>
Oddly enough, the access log shows HTTP status 200, so I cannot grasp the problem well.
The error response can also be seen in the Firefox developer tools.
My best guess is that either Zammad (that is primarily Ruby) also has a setting like PHP’s upload_max_filesize that has to be adapted (I know nothing about Ruby); or that my configuration somehow struggles with the HTTP request header line Expect: 100-continue (but why should all Web browser and curl fail?).