Connection to the Qlik Sense engine failed for unspecified reasons randomly on the hub
Connection to the Qlik Sense engine failed for unspecified reasons is displayed randomly on the hub or while using an app. When several tabs are opened at the same time, the error appears on one of the tab but not on others, showing that this is not a general loss of connection. Another symptom is when selecting a data source (to attach files), it is throwing the error dialog saying "Data Profilling connection failed. Refresh your browser."
Qlik Sense Enterprise less than version 3.2 (it seems in some case it helps even in latest versions September 2018 and above)
In 3.0 Qlik Sense begun supporting IPv6. In some environments, where IPv6 is turned off on the NIC, Qlik Sense will try to bind to IPv6 and fail. The change to the Engine's settings file disables IPv6 usage.