I have web-function that works perfectly in SQL Anywhere 16.0.0.2546:

create function "fa_get_info"( 
  in "urlSpec" long varchar,
  in "headerSpec" long varchar,
  in "certificate1stav" long varchar ) 
returns long varchar
url '!urlSpec'
type 'HTTP:POST:application/json'
certificate '!certificate1stav'
set 'REDIR (COUNT = 0)'
header '!headerSpec'

In SQL Anywhere 17.0.10.6057, I sometimes get a response from this function, and sometimes an exception.
"-zoc"-log does not contain an error.
The function uses the same certificate.

OK: BODY={"success":true,"username":"...}

[connid = 1, [10/04/2020:16:44:29.036 0300], REQUEST]
POST /login/ajaxSuccess HTTP/1.0
Date: Fri, 10 Apr 2020 13:44:29 GMT
Connection: close
Content-Type: application/json; charset=windows-1251
ASA-Id: ...
COOKIE: JSESSIONID=...
Content-Length: 0
Host: d4wnew.1stAvailable.com.au
User-Agent: SQLAnywhere
Accept-Charset: windows-1251, UTF-8, *

[connid = 1, [10/04/2020:16:44:29.896 0300], RESPONSE]
HTTP/1.1 200 OK
1st_backend: prod_app22
Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers: x-prototype-version,x-requested-with
Access-Control-Allow-Methods: GET,POST,OPTIONS
Access-Control-Allow-Origin: *
Content-Security-Policy-Report-Only: ...
Content-Type: application/json;charset=UTF-8
Date: Fri, 10 Apr 2020 13:44:27 GMT
p3p: CP="CAO PSA OUR"
Server: Apache-Coyote/1.1
Strict-Transport-Security: max-age=31536000
Content-Length: 143
Connection: Close
{"success":true,"username":"...}
[connid = 1, socket closed]

EXCEPTION: -990, WW059, The secure connection to the remote host failed: The connection was closed from the other side

[connid = 1, [10/04/2020:16:54:38.782 0300], REQUEST]
POST /appointmentBook/list/2102?format=json&all=true HTTP/1.0
User-Agent: SQLAnywhere
Content-Length: 0
Date: Fri, 10 Apr 2020 13:54:38 GMT
Host: d4wnew.1stAvailable.com.au
ASA-Id: ...
X-Requested-With: XMLHttpRequest
Connection: close
Accept-Charset: windows-1251, UTF-8, *
COOKIE: JSESSIONID=...
Content-Type: application/json; charset=windows-1251

[connid = 1, [10/04/2020:16:54:39.725 0300], RESPONSE]
HTTP/1.1 200 OK
1st_backend: prod_app22
Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers: x-prototype-version,x-requested-with
Access-Control-Allow-Methods: GET,POST,OPTIONS
Access-Control-Allow-Origin: *
Content-Security-Policy-Report-Only: ...
Content-Type: application/json;charset=UTF-8
Date: Fri, 10 Apr 2020 13:54:36 GMT
p3p: CP="CAO PSA OUR"
Server: Apache-Coyote/1.1
Strict-Transport-Security: max-age=31536000
Connection: Close
[{"id":5228,"subscriberName":"Dr ...}]
[connid = 1, socket closed]

Why so? Bug or ... ?