Solution : https://service.sap.com/sap/support/notes/1575739 (SAP Service marketplace login required)
Key words : 
threadid=449137&tstart=0 downloading sap gui patches, /irj/sdn/sap-gui, profile parameter snc/force_login_screen = 1, prerequisites program error solution, single snc-user-mapping, sap gui installation guide, terms sap gui, snc/force_login_screen = 1 reason, sap gui patches, sap kernel patchlevel 111
Related Notes : 
       
| 1053737 | Expected release dates for SAP GUI for Windows & ITS Patches | 
| 563161 | SAP GUI, SAP ITS & SAP IGS patches on Service Marketplace | 
| 535308 | How to apply a SAP Front End patch? | 
| 330793 | Downloading patches |