WORKAROUND
-> jsf-extensions
-> run-time
-> avatar
-> com.sun.faces.extensions.avatar.lifecycle.AsyncResponse
...
public void installOnOffResponse(FacesContext context) {
// my workaround - I WOULD LIKE TO REMOVE IT AS SOON AS THE ISSUE IS
SOLVED
context.getExternalContext().setResponseCharacterEncoding("UTF-8");
...