Apache Returns 403 forbidden

Question

I have API Project written in PHP 7.0.22 using Slim 3.0 framework. The app API is running on AWS (EC2) server using the elastic load balancer with Ubuntu 14.04.5, Apache 2.4.7.

Most of the time the requests return a valid response but some of them return a "403 forbidden" (the same requests that return a valid response)

I checked the Apache logs but there's nothing there. I think the blockage comes from the server.

I tried to find out if there is any kind of module which can cause this. but i did not find the problem. list of modules :

Array
(
    [0] => core
    [1] => mod_so
    [2] => mod_watchdog
    [3] => http_core
    [4] => mod_log_config
    [5] => mod_logio
    [6] => mod_version
    [7] => mod_unixd
    [8] => mod_access_compat
    [9] => mod_alias
    [10] => mod_auth_basic
    [11] => mod_authn_core
    [12] => mod_authn_file
    [13] => mod_authz_core
    [14] => mod_authz_host
    [15] => mod_authz_user
    [16] => mod_autoindex
    [17] => mod_deflate
    [18] => mod_dir
    [19] => mod_env
    [20] => mod_evasive20
    [21] => mod_filter
    [22] => mod_mime
    [23] => prefork
    [24] => mod_negotiation
    [25] => mod_php5
    [26] => mod_rewrite
    [27] => mod_setenvif
    [28] => mod_socache_shmcb
    [29] => mod_ssl
    [30] => mod_status
)

Where can this blockage come from?