I am still debugging a bit:

2017/01/10 18:17:59 [debug] 5174#5174: accept mutex lock failed: 0
2017/01/10 18:17:59 [debug] 5174#5174: epoll timer: 500
2017/01/10 18:17:59 [debug] 5172#5172: epoll: fd:13 ev:0005 d:00007F81B6D351D0
2017/01/10 18:17:59 [debug] 5172#5172: *1 http keepalive handler
2017/01/10 18:17:59 [debug] 5172#5172: *1 malloc: 00007F81B7A8B320:1024
2017/01/10 18:17:59 [debug] 5172#5172: *1 SSL_read: 461
2017/01/10 18:17:59 [debug] 5172#5172: *1 SSL_read: -1
2017/01/10 18:17:59 [debug] 5172#5172: *1 SSL_get_error: 2
2017/01/10 18:17:59 [debug] 5172#5172: *1 reusable connection: 0
2017/01/10 18:17:59 [debug] 5172#5172: *1 posix_memalign: 00007F81B7A98570:4096 @16
2017/01/10 18:17:59 [debug] 5172#5172: *1 event timer del: 13: 1484065126843
2017/01/10 18:17:59 [debug] 5172#5172: *1 http process request line
2017/01/10 18:17:59 [debug] 5172#5172: *1 http request line: "GET /SOGo/ HTTP/1.1"
2017/01/10 18:17:59 [debug] 5172#5172: *1 http uri: "/SOGo/"
2017/01/10 18:17:59 [debug] 5172#5172: *1 http args: ""
2017/01/10 18:17:59 [debug] 5172#5172: *1 http exten: ""
2017/01/10 18:17:59 [debug] 5172#5172: *1 posix_memalign: 00007F81B7A8D750:4096 @16
2017/01/10 18:17:59 [debug] 5172#5172: *1 http process request header line
2017/01/10 18:17:59 [debug] 5172#5172: *1 http header: "Host: email_server.domain.tld:port_number"
2017/01/10 18:17:59 [debug] 5172#5172: *1 http header: "Connection: keep-alive"
2017/01/10 18:17:59 [debug] 5172#5172: *1 http header: "Cache-Control: max-age=0"
2017/01/10 18:17:59 [debug] 5172#5172: *1 http header: "Upgrade-Insecure-Requests: 1"
2017/01/10 18:17:59 [debug] 5172#5172: *1 http header: "User-Agent: Mozilla/5.0 (Linux; Android 6.0.1; Nexus 5 Build/MMB29T) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.85 Mobile Safari/537.36"
2017/01/10 18:17:59 [debug] 5172#5172: *1 http header: "Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8"
2017/01/10 18:17:59 [debug] 5172#5172: *1 http header: "Accept-Encoding: gzip, deflate, sdch, br"
2017/01/10 18:17:59 [debug] 5172#5172: *1 http header: "Accept-Language: fr-FR,fr;q=0.8,en-US;q=0.6,en;q=0.4"
2017/01/10 18:17:59 [debug] 5172#5172: *1 http header done
2017/01/10 18:17:59 [debug] 5172#5172: *1 generic phase: 0
2017/01/10 18:17:59 [debug] 5172#5172: *1 rewrite phase: 1
2017/01/10 18:17:59 [debug] 5172#5172: *1 test location: "/"
2017/01/10 18:17:59 [debug] 5172#5172: *1 using configuration "/"
2017/01/10 18:17:59 [debug] 5172#5172: *1 http cl:-1 max:1048576
2017/01/10 18:17:59 [debug] 5172#5172: *1 rewrite phase: 3
2017/01/10 18:17:59 [debug] 5172#5172: *1 post rewrite phase: 4
2017/01/10 18:17:59 [debug] 5172#5172: *1 generic phase: 5
2017/01/10 18:17:59 [debug] 5172#5172: *1 generic phase: 6