While accessing the Qlik Sense Hub you are unable to connect and get this error "Cannot establish a connection: Qlik Sense G3 Broker API"
Qlik Sense 3.x and later
It appears that this error can occur when running in complex environment based on security rule and a large number of custom properties such as AD group attributes.
The postgres Database gets overloaded slows down to respond and therefore causes connection issues
Verify that all ports needed by the system are available. Review the official Qlik Help
site for an updated list.
For example, port 9028 (Broker service, used Internal only) could be used by another software.
If ports are available, to resolve this issue you can change the value set to the setting called "ReadAccessControlMode" (by default stored in C:\Program Files\Qlik\Sense\Repository\Repository.exe.config)
This settings governs what tools Qlik Sense uses to process the various security and sync rules to determine access to applications and resources within Qlik Sense. By default it is set to EntityFrameworkFiltering, which is an older more generic framework which handles large numbers of applications quite well but can run slowly if you have a large set of complicated rules.
To make the change simply:
1. Edit the Repository.exe.config file in your Qlik Sense install directory (by default C:\Program Files\Qlik\Sense\Repository)
2. Find ReadAccessControlMode
3. Change the value to "RepositoryFiltering"
4. Restart Qlik Sense Services
In a multi node environment we recommend to apply this setting on every node.