QGIS as OGC Data Server only returns white box

The GetCapabilities response seems to be valid, and you get a response to the request so the WMS service appears to be working as expected.

The GetMap request you've made (or reported in the question) is invalid as it's missing required parameters, I'm surprised you get any image response as the default error is XML unless you specify differently using the EXCEPTIONS parameter.

You are missing styles (either like styles& or styles=default& or specified in and SLD parameter...), version (I assume version=1.3.0& as you have CRS), bbox (min lat, min lon, max lat, max long for EPSG:4326), output format (like format=image/png&).

Assuming/once you have a correct request then a white image response (with no error text) might not mean you have a bad response, just your bounding box describes an area of no data.

Rather than hand cranking GetMap requests to test a service, why not instead load the WMS into QGIS desktop, specifying the endpoint for the GetCapabilities request in this case http://localhost:8080/qgisserver?MAP=/projects/example/world3.qgs& then zoom to extent.

I also faced this problem and a solution is very easy but doesn't mention on official docs. You need to specify bbox parameter on url.

For the mentioned world.qgs project try:

BBOX=-25345974.386137474%2C16185170.55818786%2C34789346.44239271%2C23844254.527065746