Max,
There are at least two different versions of Xloader, one is for model airplanes and one for general purpose ATMEGA328p loading.? You need the general purpose one.? I found it here:
There are other locations, you need to ensure that one of the choices is the Uno(ATmega328) as well as having the driver loaded (since you can load Blink, then the driver is loaded).
Try that and see if the Nextion screen updates.? The symptoms that you are describing agree with what I found when using the wrong Xloader.
73
Evan
AC9TU
