I have already have this setting done changing into legacy mode on my local machine.
This error I am getting on the server only.
If app is run on local machine it's working fine with UNC path but if it's scheduled on the server it stops with error given above.
I should say that server is having access to the path and definitely tested.
In a QlikSense server you also have to disable standard mode.
When selected, standard mode is used. If cleared, legacy mode is used. Standard mode is the default mode that prevents actions that are potentially harmful. Standard mode is to be used unless there are special reasons not to. Legacy mode can be used for running QlikView load scripts unchanged, when loading data into Qlik Sense.
For security reasons, Qlik Sense in standard mode does not support absolute or relative paths in the data load script or functions and variables that expose the file system.