WMS layer is blank because the server returns ServiceException XML

• The response body is ServiceExceptionReport with code LayerNotDefined and text: Layer not defined: roads.
• Content-Type is application/vnd.ogc.se_xml instead of image/png.
• Overlaid on the basemap the layer shows nothing, or covers the basemap with an opaque block when JPEG is used.

1. Copy the failing GetMap URL and open it directly in the browser.
2. Check the response Content-Type before looking at pixels.
3. Check LAYERS for the workspace prefix, for example workspace:roads.
4. Check VERSION against the axis parameter: 1.1.1 uses SRS, 1.3.0 uses CRS with lat-first BBOX.

Build the source with LAYERS 'workspace:roads', STYLES '', VERSION '1.1.1', SRS 'EPSG:3857', FORMAT 'image/png' and TRANSPARENT true; for 1.3.0 use CRS and let OpenLayers order the BBOX axes.

<?xml version="1.0" encoding="UTF-8"?>
<ServiceExceptionReport version="1.3.0">
  <ServiceException code="LayerNotDefined">
    Layer not defined: roads
  </ServiceException>
</ServiceExceptionReport>

map.getLayers().forEach(function (l) {
  var src = l.getSource();
  if (src instanceof ol.source.TileWMS) {
    console.log(src.getFeatureInfoUrl(
      map.getView().getCenter(),
      map.getView().getResolution(),
      'EPSG:3857',
      { INFO_FORMAT: 'text/html' }
    ));
  }
});

• Using FORMAT image/jpeg while requesting TRANSPARENT=true still paints an opaque rectangle over the basemap.
• Omitting the workspace prefix works only when the layer sits in the default workspace.