Webserver Stress Tool: error while testing https url

When I test my API with https protocol I receive an error. The url is correct and I can see the result of a simulation using the "URL recorder" tools.

6/01/2017 10:14:27: 1 URLs
16/01/2017 10:14:27:   URL#1: GET https://www.****.com/php/getSongs.php?local_id=***** POSTDATA= Click Delay=
16/01/2017 10:14:27:  
16/01/2017 10:14:27: ** Browser Settings **
16/01/2017 10:14:27: Browser Simulation: 
16/01/2017 10:14:27:   User Agent: Mozilla/5.0 (compatible; Webserver Stress Tool 8; Windows)
16/01/2017 10:14:27:   HTTP Request Timeout: 120 s
16/01/2017 10:14:27:  
16/01/2017 10:14:27: Recursive Browsing / HTML Parsing: 
16/01/2017 10:14:27:  
16/01/2017 10:14:27: ** Options **
16/01/2017 10:14:27: Advanced Settings: 
16/01/2017 10:14:27: Logging:
16/01/2017 10:14:27:   Write detailed log(s)
16/01/2017 10:14:27: Timer: not enabled
16/01/2017 10:14:27:  
16/01/2017 10:14:27: Using Local IPs: 
16/01/2017 10:14:27:   172.16.4.29
16/01/2017 10:14:27: Advanced Data Merging Features:
16/01/2017 10:14:27:  
16/01/2017 10:14:27: ** Client System **
16/01/2017 10:14:27:   Windows 8/2012 V6.2 (Build 9200) , CPU Proc. Lev. 686 (Rev. 24067) at 3.408 MHz, 
16/01/2017 10:14:27:   12901 MB available RAM of 17056 MB total physical RAM, 15616 MB available pagefile
16/01/2017 10:14:27:  
16/01/2017 10:14:27: Test is starting
16/01/2017 10:14:27: ----------------------------------------------------
16/01/2017 10:14:27: Creating Users...
16/01/2017 10:14:27: Test preparations done
16/01/2017 10:14:27: 
16/01/2017 10:14:27: Master Controller will start now...
16/01/2017 10:14:28: Switching to 1 users.
16/01/2017 10:14:28: User #1 : !FAILED! CLICK-Request 1: Time=379 ms, TFB=0 ms, Bytes=0, HTTP-StatusCode=0 ((unknown http result)) URL #1=https://www.******.com/php/getSongs.php?local_id=*****