Are you able to authenticate with qlik sense server? what is the authentication method you have? did you look into your network log in your browser? Post the screenshots of the error and the network tab from browser dev tools.
ErrorCode.16 translates into "Connection lost. Make sure that Qlik Sense is running properly. If your session has timed out due to inactivity, refresh to continue working.", you should have gotten the translated string instead of the cryptic error code.
What Qlik Sense version are you using? And can you share your code?