Why I have orange triangle with exclamation mark in the top left corner of the screen

[AC Question]

Hi everybody!
I am using s4 mini and it shows orange triangle with exclamation mark in the top left corner of the screen, I understand it is some sort of error warning but can't localise it, I know it is not related to WI-FI, data or battery issues. How do I find out what is wrong, or how I troubleshoot this. Any ideas will be appreciated.