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.
I'm going through the same problem they had, I mark the same error and I can no longer access Sense QMC. Try searching for pgAdmin.exe but I can not find that .exe in the path I specify Chadwick Leung.
The second step I do not understand how it is done.
Ariane Gadd I do not understand how to follow the steps you took to solve the problem, you could help me please or tell me where I need to go.
is that I'm just using QlikSense Server and I'm still not familiar with the Server
I hope you can help me
The mentioned pgAdmin3.exe path refers to the Qlik Sense 3.2.x installation. If you have Jun 2017 (or later) Release installed, the pgadmin program is not bundled. You need to download and install it yourself, the binary can be found from pdadmin.org site. I forgot which versions I had problem to make a connection, the version 4 v1.1 has worked fine.
The default path of the pgadmin installation is
"C:\Program Files (x86)\pgAdmin 4\v1\runtime\pgAdmin4.exe"
After you started the pgadmin4 program, create a new "server" connection according to the properties of your environment (as the step 2 shown and follow the steps to locate the VirtualProxyConfigs table for an update.
Hope this help. Good Luck