I finally found a workaround by restoring the value of the Anonymous access mode at DB level (PostgreSQL).
1) Execute the pgAdmin program (path ..\Qlik\Sense\Repository\PostgreSQL\9.3\bin\pgAdmin3.exe) and connect to the DB. Refer to the attached image, the password is the one input during QS server installation
2) Locate the table "VituralProxyConfigs". For my case, the table is under [QlikSenseDB] -> [Databases] -> [QSR] -> [Schemas] -> [public] -> [Tables] -> [VirtualProxyConfigs].
3) When you view the data in the table, refer to the column [AnonymoseAccessMode] the value for the "Central Proxy (Default)" row. To restore to the option a (as below), update the value from 2 to 0.
a) No anonymous user 
b) Allow anonymous user 
c) Always anonymous user 
Hope this help.