see this suggestion
and these answers
May the Force be with you
I'm sure that your configuration was in legacy mode but pay attention.
There are some astral condition in Qlik Sense (Server) that could lead you to loose this setting :
- configure Qlik Sense (Server) Engine in legacy mode
- restart all Qlik Sense services
- there is possibility that you loose this setting
Could you post some logs or provide more information?
P.S. : May the Force be with you