Fix: EventSource’s response has a charset (“iso-8859-1”) that is not UTF-8. Aborting the connection.

The issue is with the mismatch charset. You have default charset charset : iso-8859-1 and you are trying to use content which have charset=UTF-8. To fix the error you need to set the charset=UTF-8 in which you have set the header. In my case, I have fix the error by setting the charset=UTF-8 in header.